Showing papers on "Antibody published in 1981"

Prediction of protein antigenic determinants from amino acid sequences.

Abstract: A method is presented for locating protein antigenic determinants by analyzing amino acid sequences in order to find the point of greatest local hydrophilicity. This is accomplished by assigning each amino acid a numerical value (hydrophilicity value) and then repetitively averaging these values along the peptide chain. The point of highest local average hydrophilicity is invariably located in, or immediately adjacent to, an antigenic determinant. It was found that the prediction success rate depended on averaging group length, with hexapeptide averages yielding optimal results. The method was developed using 12 proteins for which extensive immunochemical analysis has been carried out and subsequently was used to predict antigenic determinants for the following proteins: hepatitis B surface antigen, influenza hemagglutinins, fowl plague virus hemagglutinin, human histocompatibility antigen HLA-B7, human interferons, Escherichia coli and cholera enterotoxins, ragweed allergens Ra3 and Ra5, and streptococcal M protein. The hepatitis B surface antigen sequence was synthesized by chemical means and was shown to have antigenic activity by radioimmunoassay.

Adult T-cell leukemia : antigen in an ATL cell line and detection of antibodies to the antigen in human sera

Abstract: Indirect immunofluorescence of certain human sera demonstrated an antigen(s) in the cytoplasm of 1--5% of the cells of a T-cell line, MT-1, from a patient with adult T-cell leukemia (ATL), which is endemic in southwestern Japan. The antigen was not detected in other human lymphoid cell lines, including six T-cell lines, seven B-cell lines, and four non-T non-B cell lines. The antigen did not show cross antigenicity with that of herpesviruses, including Epstein--Barr virus, herpes simplex virus, cytomegalovirus, varicella-zoster virus, herpesvirus saimiri, and Marek disease virus. The proportion of antigen-bearing cells was increased by a factor of approximately 5 on culture in the presence of 5-iodo-2'-deoxyuridine. Antibodies against the antigen in MT-1 cells were found in all 44 patients with ATL examined and in 32 of 40 patients with malignant T-cell lymphomas (most of them had diseases similar to ATL except that leukemic cells were not found in the peripheral blood). The antibodies were also detected in 26% of the healthy adults examined from ATL-endemic areas but in only a few of those examined from ATL-non-endemic areas. On electron microscopy, extracellular type C virus particles were detected in pelleted MT-1 cells cultured in the presence of 5-iodo-2'-deoxyuridine.

Preparation of monoclonal antibodies: strategies and procedures.

Abstract: Publisher Summary This chapter discusses the strategies and procedures for the preparation of monoclonal antibodies (McAb). The derivation of permanent lines of hybrid cells, producing McAb, exhibiting certain desired properties, presents widely different degrees of difficulty. Desired properties include not only specific recognition of an antigen and other no less critical properties are the fine specificity of the antibody, avidity and kinetic parameters important for radioimmunoassays, cytotoxic properties necessary for direct complement-dependent lysis. The insoluble antigen and the antibody in the culture fluid are allowed to react. The free antibody is washed away. The amount of monoclonal antibody bound is measured directly or by binding of a second, labeled antibody capable of recognizing the first. Monoclonal antibodies can be easily labeled internally at high specific activity, using radioactive amino acid precursors. The choice of these is based on the efficiency of incorporation of labeled amino acids into secreted immunoglobulin in culture conditions. The hemagglutination assays are based on the ability of an antibody to agglutinate red cells, carrying the specific antigen. These assays have all the advantages in terms of extreme simplicity, speed, and direct visual reading of the results.

F4/80, a monoclonal antibody directed specifically against the mouse macrophage

Abstract: A hybridoma clone which secretes a macrophage (MΦ)-specific monoclonal antibody, F4/80, was produced by fusing spleen cells from a rat hyperimmunized with cultured thioglycollate-induced mouse peritoneal MΦ with a mouse myeloma, NS1. Binding of antibody to primary cells and cell lines was detected by radioimmune indirect binding assay, autoradiography or fluorescence-activated cell sorter analysis. F4/80 binds to mouse MΦ from the peritoneal cavity or other sources, blood monocytes, MΦ derived from bone marrow precursors in culture and MΦ-like cell lines, but not to other cells, including polymorphonuclear leukocytes, lymphocytes or fibroblasts. F4/80 does not bind to MΦ via Fc receptors, is not cytotoxic and is of the rat IgG2b subclass. Since F4/80 binds to all MΦ defined by adherence, morphology and immune phagocytosis, it provides a new marker to define the MΦ in the mouse. Large differences in expression of antigen F4/80 were found, depending on intraperitoneal stimulation, time in culture and stage of maturation. Immunoprecipitation experiments demonstrated that the antigen F4/80 is part of a component of Mr 160000 which is synthesized by the MΦ and, at least in part, exposed on the cell surface.

Monoclonal antibodies specific for simian virus 40 tumor antigens.

Abstract: Thirty hybridomas that secrete immunoglobulins against the simian virus 40 tumor antigens were isolated and cloned. Of these, 28 produced antibodies which bound to simian virus 40 large-T, and 2 produced antibodies which bound to the host 53,000-dalton protein. As in previous work, large-T antigen was found to have at least one determinant that it shared with small-t antigen and to have a minimum of six unique determinants. Several of the monoclonal antibodies from the L series hybridomas recognized determinants that were present on a subset of the large-T antigen from simian virus 40-transformed mouse cells. These monoclonal antibodies should be useful in studies of the structure and function of the simian virus 40 tumor antigens.

Ubiquitous cell-surface glycoprotein on tumor cells is proliferation-associated receptor for transferrin.

Abstract: A murine monoclonal antibody (OKT9) raised against human leukemic cells binds to a wide variety of leukemia and tumor cell lines and to a minority of leukemia cells taken directly from patients Fetal thymus and liver are strongly reactive as are some normal, immature hemopoietic cells and activated lymphocytes Reactivity with OKT9 appears to correlate with proliferation status in both normal and malignant populations Biochemical analysis indicates that this structure is a approximately equal to 180,000-dalton glycoprotein with two disulfide-bonded subunits of approximately equal to 90,000-daltons Isolation of the transferrin receptor from a T-cell line (MOLT-4) indicates that it also has a dimeric approximately equal to 180,000-dalton structure Radio-labeled transferrin bound to its receptors can be specifically precipitated by the monoclonal OKT9, although the latter does not bind transferrin itself, indicating that the antigenic structure defined by this antibody is likely to be the transferrin receptor

Human cell surface glycoprotein related to cell proliferation is the receptor for transferrin.

Abstract: A cell surface glycoprotein antigen with an apparent molecular weight of about 100,000 that is selectively expressed on proliferating cells was purified from deoxycholate-solubilized membranes of a cultured human leukemic thymus-derived (T) cell line by affinity chromatography on a monoclonal antibody-Sepharose column. A conventional xenoantiserum prepared by immunization with the affinity-purified glycoprotein was found to contain antibodies against a serum component that bound tightly to cultured cells. This molecule was shown to be specifically associated with the cell surface glycoprotein purified by immunoprecipitation from lysates of cells. We have identified the serum component as transferrin and conclude that the membrane glycoprotein is the cell surface transferrin receptor.

Monoclonal antibodies to nucleic acid-containing cellular constituents: probes for molecular biology and autoimmune disease

Abstract: Mice of the strain MRL/Mp-lpr/lpr develop a lupus erythematosus-like syndrome that includes the production of autoantibodies specific for nucleic acid-containing cellular components. We have fused spleen cells from such a mouse with the myeloma SP 2/0 and examined the antibodies produced by the resultant cloned hybrid cell lines by using immunoprecipitation and immunofluorescence techniques. Three types of monoclonal antibodies, specific for Sm, DNA, or rRNA, all antigens to which patients who have lupus make antibodies, have been identified. Patient anti-Sm antibody had previously been reported to precipitate five small nuclear ribonucleoproteins that contain U-1, U-2, U-4, U-5, and U-6 RNAs. The monoclonal anti-Sm antibody gives the same immunoprecipitation pattern, providing direct evidence that the Sm antigen resides on all these RNA-protein complexes. Monoclonal anti-Sm antibody will be valuable in deciphering the biological function of these ubiquitous small nuclear RNPs. A simple competition radioimmunoassay using the monoclonal anti-Sm antibody to titer patient sera is also presented. Uses of monoclonal antibodies for the study of autoimmune disease are discussed.

Stage-specific embryonic antigen involves alpha 1 goes to 3 fucosylated type 2 blood group chains.

Abstract: There is much interest in developmentally regulated molecules which may have function in cell interactions and sorting during embryogenesis and differentiation. Numerous antisera have been raised which detect antigens that are expressed in early embryonic cells and become restricted during differentiation, being expressed in only a minority of adult cells (reviewed in refs 1–3). The precise antigenic determinants recognized by such antisera have not been defined. However, studies using a hybri-doma antibody against mouse spleen cells4 and monoclonal autoantibodies of patients with cold agglutinin disease5 have shown that two defined carbohydrate antigen systems, the Forssman and the li antigens, have stage-specific expression in early mouse embryos. We now describe evidence that the stage-specific embryonic antigen SSEA-1 (ref. 6) involves the carbohydrate sequence Galβ1 → 4GlcNAc ↑1,3 Fucα This determinant is formed by α 1 → 3 fucosylation of blood group I or i antigens which are branched or linear oligosaccharides, respectively7–9, built of Galβ1 →4GlcNAc units and known as type 2 precursor chains10 of the major blood group antigens. Thus, we introduce the concept of simple glycosylation changes as a basis for stage-specific expression of embryonic antigens.

Monoclonal antibodies to epithelium-specific components of the human milk fat globule membrane: production and reaction with cells in culture.

Abstract: Three hybridomas producing monoclonal antibodies (IgG), reacting with components of the human mammary milk fat globule have been isolated. When tested for binding to a wide range of human cell lines and strains, all three antibodies show negative reactions with fibroblasts, lymphoblastoid cells, and a large number of epithelial cell lines of non-breast origin. Two of the antibodies (1.10.F3 and 3.14.A3) reacted with seven out of eight breast cancer lines tested, and with epithelial cells cultured from human milk. The other antibody (3.15.C3) reacted with only two of the breast cancer cell lines.

A spectrum of monoclonal antibodies reactive with human mammary tumor cells

Abstract: Splenic lymphocytes of mice, immunized with membrane-enriched fractions of metastatic human mammary carcinoma tissues, were fused with the NS-1 non-immunoglobulin-secreting murine myeloma cell line. This resulted in the generation of hybridoma cultures secreting immunoglobulins reactive in solid-phase radioimmunoassays with extracts of metastatic mammary carcinoma cells from involved livers, but not with extracts of apparently normal human liver. As a result of further screening of immunoglobulin reactivities and double cloning of cultures, 11 monoclonal antibodies were chosen that demonstrated reactivities with human mammary tumor cells and not with apparently normal human tissues. These monoclonal antibodies could be placed into at least five major groups on the basis of their differential binding to the surface of various live human mammary tumor cells in culture, to extracts of mammary tumor tissues, or to tissue sections of mammary tumor cells studied by the immunoperoxidase technique. Whereas a spectrum of reactivities to mammary tumors was observed with the 11 monoclonal antibodies, no reactivity was observed to apparently normal cells of the following human tissues: breast, lymph node, lung, skin, testis, kidney, thymus, bone marrow, spleen, uterus, thyroid, intestine, liver, bladder, tonsils, stomach, prostate, and salivary gland. Several of the antibodies also demonstrated a "pancarcinoma" reactivity, showing binding to selected non-breast carcinomas. None of the monoclonal antibodies showed binding to purified ferritin or carcinoembryonic antigen. Monoclonal antibodies of all five major groups, however, demonstrated binding to human metastatic mammary carcinoma cells both in axillary lymph nodes and at distal sites.

IgE‐induced histamine release from rat basophilic leukemia cell lines: isolation of releasing and nonreleasing clones

Abstract: The rat basophilic leukemia (RBL) cell lines were cloned and the various sublines compared for their chromosome number, IgE-mediated histamine release and for IgE surface receptors. It was found that cell lines started from tumors at different times vary in both their chromosome number and their ability to release histamine by an IgE-mediated reaction. RBL-I and III have approximately 44 chromosomes and did not respond to an IgE-mediated reaction. RBL-II and RBL-IV have 68-73 chromosomes and showed moderate levels of histamine release (percent release mean = 5 +/- 2 and 10 +/- 4, respectively). The cloning of the RBL-IV line resulted in some sublines which were excellent histamine releasers (range 39-100%) and some which were relatively refractory (less than 10%) to IgE-mediated histamine release. These clones did not differ significantly in chromosome number. Recloning the releasing lines gave rise to poor releasers, whereas the recloning of poor releasers did not produce good releasers indicating that the mutational drift in culture is toward loss of histamine-releasing capacity. The number of IgE receptors and the rate of IgE association and dissociation were similar for the different cell lines. The study failed to disclose significant molecular weight differences in the IgE receptor from the various clones and sublines indicating that the failure to release probably does not reside in the receptor. The various cloned sublines are phenotypically stable, and the isolation of excellent histamine-releasing sublines are useful for studies of the complex phenomenon of the histamine release.

Monoclonal antibodies to mouse MHC antigens. III. Hybridoma antibodies reacting to antigens of the H-2b haplotype reveal genetic control of isotype expression.

Abstract: Eleven hybridoma antibodies directed against mouse major histocompatibility complex products of the H-2b haplotype have been produced and characterized. Of 7 antibodies reacting to H-2Kb and/or H-2Db antigens, all cross-reacted with other H-2 antigens, and 5 exhibited no correspondence with a known H-2 specificity established in the H-2 chart. Four anti-Iab antibodies all reacted with antigens encoded by the I-A subregion. Some of these antibodies showed no cross-reaction with other haplotypes, indicating reactions to private specificities of the I-Ab antigen. In addition, these anti-Ia antibodies appeared to be capable of distinguishing fine determinant differences, which corresponding alloantisera failed to reveal. A high frequency of hybridomas secreting IgM antibodies was found after fusions of spleen cells obtained from C3H anti-C3H.SW immunized mice, in contrast to the dominance of IgG hybridomas produced previously by fusions of spleen cells from mice immunized in the reverse direction. An isotype analysis of conventional cytotoxic alloantisera from the same strain combinations was therefore performed. The same correlation with respect to isotype expression was found, indicating that hybridoma antibodies reflect normal antibody responses and suggesting H-2-linked control of this expression.

Reactivity of Langerhans cells with hybridoma antibody

Abstract: Reactivity of a monoclonal antibody with human Langerhans cells was demonstrated by a double-labeling immunofluorescence technique. Ia-bearing cells of the epidermis (Langerhans cells) were reactive with this antibody both in frozen sections and in cell suspensions prepared from human epidermis. This monoclonal antibody was unreactive with non-Ia-bearing epidermal cells and with peripheral blood B cells, T cells, and monocytes but did not bind to 70% of intrathymic lymphocytes. These observations further distinguish Langerhans cells from classical monocytes. Furthermore, this monoclonal antibody is a highly specific marker for the in vivo identification and in vitro isolation of Langerhans cells.

A shared alloantigenic determinant on Ia antigens encoded by the I-A and I-E subregions: evidence for I region gene duplication.

Abstract: Two monoclonal antibodies to mouse Ia antigens were produced by fusion of xenoimmune rat spleen cells with the NSI myeloma. These monoclonal antibodies detect polymorphic determinants present on B cells and activated T lymphocytes from mice carrying the H-2b, H-2d, H-2k, H-2r, and H-2q haplotypes but not from mice carrying the H-2s or H-2r haplotypes. Antigenic site number determinations showed the positive haplotypes can be divided into 2 groups. Mice bearing the H-2b, H-2d, and H-2q haplotypes express a high number--40,000 to 80,000--of antigenic sites per B lymphocyte, and monoclonal antibody plus complement can lyse B cells from these mice. In contrast, mice bearing the H-2k and H-2r haplotypes express a low number of antigenic sites--about 5000 per cell. Spleen cells from mice carrying the latter haplotypes are not lysed with monoclonal antibody and complement. Genetic mapping demonstrated that high and low expression map to the I-A and I-E subregions, respectively. The monoclonal antibodies detect an Ia specificity on I-Ab, I-Ad, I-Ed, and I-Ek molecules. These observations were confirmed using several different experimental approaches, i.e., cytotoxicity, fluorescent staining, competitive inhibition of monoclonal antibody binding, and 2-dimensional gel electrophoresis of immunoprecipitates. The avidity for A alpha b A beta b and E alpha k E beta k is 5 to 7 x 10(-9) M-1. The antigenic determinant is heat labile, which suggests that it is not carbohydrate. The results imply that Ia antigens encoded by distinct subregions share sequence homology, which may be a consequence of ancestral gene duplication.

Chronic idiopathic thrombocytopenic purpura.

Abstract: CHRONIC idiopathic (or immune) thrombocytopenic purpura (ITP) is a syndrome characterized by persistent thrombocytopenia caused by a circulating antiplatelet factor that results in platelet destruction by the reticuloendothelial system. It seems likely that the antiplatelet factor in most patients is an IgG antibody directed toward a platelet-associated antigen, although circulating immune complexes may have a role in some cases. This syndrome has been termed "autoimmune" by some authors, but this designation seems premature, since present evidence does not rule out a heterologous antigen as the causative agent. Characterization of the antigen (or antigens) will be necessary to establish true autoimmunity. . . .

A monoclonal antibody discriminating between subsets of T and B cells.

Abstract: A description is given of a rat anti-mouse hybridoma antibody, JIId, which reacts with erythrocytes, neutrophils, greater than 90% of thymus cells, and most B cells JIId does not have detectable activity for mature T cells, pluripotential stem cells, platelets, or cells of the monocyte-macrophage lineage Although the JIId antigen is present on 90 to 95% of typical small B lymphocytes, pretreatment of spleen cells with JIId plus complement has no effect on secondary IgG antibody responses; by contrast, primary IgM responses and proliferative responses to lipopolysaccharide are substantially reduced Unlike the precursors of IgG antibody-producing cells (AFC), IgG AFC per se are strongly JIId-positive Rapid acquisition of the JIId antigen also applied to the early progeny of pluripotential stem cells

Polyspecific monoclonal lupus autoantibodies reactive with both polynucleotides and phospholipids

Abstract: Hybridomas the produce anti-DNA autoantibodies were prepared from spleen cells of unimmunized MRL/1 mice, a strain that spontaneously develops severe systemic lupus erythematous (SLE). Reactivities of these monoclonal antibodies with a wide range of polynucleotides prompted tests of their reactions with phospholipids which, like polynucleotides, contain diester-linked phosphate groups in their backbones. In competitive radioimmunoassays, cardiolipin, phosphatidic acid, and phosphatidyl glycerol blocked the binding of these hybridoma antibodies to denatured DNA. These phospholipids also specifically inhibited the reaction between a hybridoma antibody and a site-specific anti-idiotypic antibody. The antinuclear reaction of one of these antibodies was specifically inhibited by cardiolipin. This same antibody prolonged the activated partial thromboplastin time in a manner characteristic of a lupus anticoagulant, presumably by binding to phospholipid in the test system. The polyspecific reactivity of a single molecular species of lupus autoantibody suggests that some of the diverse serological abnormalities of SLE may be a result of the binding of certain autoantibodies to a phosphodiester-containing epitope that is present in diverse biological molecules.

Antiphosphocholine antibodies found in normal mouse serum are protective against intravenous infection with type 3 streptococcus pneumoniae

Abstract: The antiphosphocholine (PC) antibody in normal mouse sera (NMS) provides protection against intravenous infection with encapsulated strain WU2 of type 3 Streptococcus pneumoniae. Mice unable to make anti-PC antibody, as a result of suppression with anti-T-15 idiotype or inheritance of the xid gene of CAB/N mice, are highly susceptible to infection with strain WU2. Mice inheriting the xid gene can be protected with NMS from immunologically normal mice or with IgM hybridoma anti-PC antibody. The protective effect of NMS can be removed with PC-containing immunoabsorbents.

Distinctive functional characteristics of human „T”︁ lymphocytes defined by E rosetting or a monoclonal anti‐T cell antibody

Abstract: The properties of human lymphocyte fractions isolated either by sheep red cell(E) rosetting or by fluorescence-activated cell sorting after staining with UCHT1 monoclonal anti-T cell antibody have been compared. Two populations of E+ cells with very different phenotype and function have been identified. E+/UCHT1+ cells respond well to the T cell mitogens phytohemagglutinin and concanavalin A and provide help for an in vitro specific antibody response. They can also suppress the antibody response of allegeneic peripheral blood mononuclear cells. In contrast, the E+/UCHT1- population, which has no other markers characteristic of T cells, fails to respond to mitogens or to provide help or suppression for an antibody response. These cells, however, are highly active natural killers. They possess Fc gamma receptors and have a characteristic staining pattern of nonspecific esterase enzyme activity. It is concluded that not all cells capable of forming E rosettes are thymus-processed cells and that this heterogeneity can be revealed by staining with the monoclonal anti-T cell reagent UCHT1.

Process for making human antibody producing B-lymphocytes

Abstract: Antibody producing human B-lymphocytes are produced by a process which comprises culturing human B-lymphocytes in a tissue culture medium in the presence of an antigen; helper signal producing agents comprising monocytes or monocyte conditioned medium containing Interleukin 1 (I1-1), and helper T-lymphocytes or helper T-lymphocyte replacing factor; and human serum; and thereafter recovering the antibody producing cells from the medium.

B220: a B cell-specific member of the T200 glycoprotein family

Abstract: T200, a major cell-surface glycoprotein on lymphoid cells, exists in several forms with different electrophoretic mobilities These forms have been correlated with different classes of lymphoid cell The smaller forms, with molecular weights (MWs) of congruent to 170,000 and 180,000 are found predominantly on T cells while the 220,000 MW form is associated with B cells The polypeptide portions of each molecule may be identical or closely related as all three forms share the same allelic variations, and all reported herologous antisera and monoclonal antibodies to T200 precipitate all three forms We report here a monoclonal antibody specific for the 220,000 MW form of T200 and show that it is expressed only on B cells and a subset of bone marrow cells which includes B cell precursors We suggest that this form of the molecule be designated provisionally B220

Characterization of a high molecular weight acidic nuclear protein recognized by autoantibodies in sera from patients with polymyositis-scleroderma overlap.

Abstract: Autoantibodies in the serum from a patient with connective tissue disease have been used to define a high molecule weight acidic nuclear protein antigen. The antigen tentatively termed Ku, after the first two letters of patient's name, has distinct physicochemical properties and immunological specificities that distinguish it from previously reported antigens. The Ku antigen has an apparent 300,000 mol wt as determined by gel filtration and sucrose density gradient ultracentrifugation techniques. The antigen is destroyed by trypsin, mild heating, and pH variations greater than 10 and less than 5. Treatment with ribonuclease or deoxyribonuclease did not affect the antigenic reactivity. The Ku antigen was demonstrated in the soluble extracts of human, calf, and rabbit, but not of rat tissues. Purified antibody localized the Ku antigen within the nuclei of human liver where a "reticular" pattern of immunofluorescence was seen. Of 330 patients with various connective tissue diseases, 9 had precipitating antibodies to the Ku antigen. Preliminary results of clinical analysis indicated that antibody to the Ku antigen might become a useful marker for a group of patients with clinical characteristics of both polymyositis and scleroderma with a good prognosis.

A unique cell surface antigen identifying lymphoid malignancies of B cell origin.

Abstract: A monoclonal antibody (anti-B1) specific for a unique B cell surface differentiation antigen was used to characterize the malignant cells from patients with leukemias or lymphomas. All tumor cells from patients with lymphomas or chronic lymphocytic leukemias, bearing either monoclonal kappa lambda light chain, expressed the B1 antigen. In contrast, tumor cells from T cell leukemias and lymphomas or acute myeloblastic leukemia were unreactive. Approximately 50% of acute lymphoblastic leukemias (ALL) of non-T origin and 50% of chronic myelocytic leukemia in blast crisis were also anti-B1 reactive. moreover, 21 of 28 patients with the common ALL antigen (CALLA) positive form of ALL were anti-B1 positive, whereas 0 of 13 patients with CALLA negative ALL were reactive. These observations demonstrate that an antigen present on normal B cells is expressed on the vast majority of B cell lymphomas and on approximately 75% of CALLA positive ALL, suggesting that these tumors may share a common B cell lineage.

Immunization against blood-stage rodent malaria using purified parasite antigens.

Abstract: We have reported the production of several hybridoma lines secreting monoclonal antibodies recognizing antigens of the blood forms of a rodent malaria parasite, Plasmodium yoelii yoelii1,2. Line WIC 25.77 secretes an IgG2a (antibody 25.77) which reacts specifically with merozoites in the indirect immunofluorescence (IIF) assay, and which was protective on passive transfer. Another of the hybridoma lines, WIC 25.1, secretes an IgG2a (antibody 25.1) which reacts with an antigen apparently associated with the membranes of schizonts, but this antibody was not protective on passive transfer. We have used our monoclonal antibodies to identify and purify antigens of P. y. yoelii, and we report here that mice immunized with either a 235,000 apparent molecular weight (MW) merozoite-specific protein or a 230,000 MW schizont protein and its proteolytic derivatives were protected against infection with P. y. yoelii.