Abstract: The initial discovery by Bednorz and Muller1 of 35-K superconductivity in the La-Ba-Cu-O system has stimulated worldwide activity in searching for higher-temperature superconductors. Elemental substitution has proved to be most effective in raising transition temperature. Substitution of Sr for Ba has produced 40-K superconductivity2–5and substitution of Y for La has produced a new high-temperature superconductor with transition temperature above liquid-nitrogen temperature6. A class of superconducting compounds of the form RBa2Cu307-x has been explored by further substitutions of other rare earths (Y is considered in the rare-earth [RI category here) for Y7-13. To date, a rare earth, an alkaline earth, copper and oxygen have been required for all high-temperature superconductors14,15. (Zhanget al 14reported 90-K superconductivity in the Th-Ba-Pb(Zr)-Cu-O system. Panetal15reported 50-K superconductivity in the Y-Ba-Ag-O system. As Th is a member of the actinide series which belongs to the same Group 3B in the periodic table as the lanthanide series and Ag belongs to the same Group 1B as Cu, high-temperature supercon-ductors are still thought to be closed in the Group 3B—Group 2A-Group 1B—oxygen system. ) Only partial substitutions ha. e led to superconductors, but with no significant rise of transition tem-perature (the only exception is 40-K superconductivity in La2CuO4-x , refs 16, 17). Here we report superconductivity in the rare-earth-free TI-Ba-Cu-O system. We have obsened sharp drops of resistance starting above 90 K with zero resistance at 81 K in this system. Magnetic measurements have confirmed that these sharp drops of resistance in the TI-Ba-Cu-O samples origi-nate from superconductivity. The samples are stable in air for at least two months, and their preparation is easily reproduced.