The specificity of the cytotoxic T lymphocyte response to Friend virus- (FV) induced leukemia was analyzed using cloned variants of the Y57 erythroleukemia cell line. As the Y57 cell line was serially passed in vitro, its susceptibility to cytotoxic T lymphocyte-mediated lysis was lost simultaneous with the spontaneous cessation of virus production. Cloned cell lines derived from Y57 differed in their susceptibility to lysis. Those clones that continued to produce virus were lysed by T lymphocytes, whereas those that had lost virus production were either lysed at a level similar to the virus-producing clones or at a greatly reduced level. Those clones showing a reduced level of direct lysis had lost cell-surface antigens recognized by T lymphocytes, since they could not block cell-mediated cytotoxicity in a cold target inhibition assay and could not stimulate FV-specific blastogenesis of nylon wool-purified T lymphocytes. By analyzing the clones for viral-specific cell surface antigens, a correlation between the presence of the viral gp70 antigen and recognition of T lymphocytes was found. This antigen and the viral p12 antigen were the only viral-specific cell surface antigens found on the clones using monospecific antiserum to the viral gp70, p30, p15, p12, and p10 antigens. Expression of viral p12 antigens did not appear to contribute to recognition by T lymphocytes, since clones that had normal amounts of this antigen, but no gp70 antigens, were not recognized in cytotoxicity or blastogenesis assays. This same pattern of recognition was also observed when the clones were compared with mouse anti-FV antibody and C. Thus, both the cellular and humoral immune responses to FV-induced leukemia appeared to be primarily directed toward the viral gp70 molecule.