The cytotoxic sensitivity of murine leukemia virus (MuLV)-infected and noninfected fibrosarcoma cells in syngeneic inbred WKA/Hok rats was compared by in vitro cell-mediated 51Cr release cytotoxicity assay. A highly significant increase in cytotoxic sensitivity of target cells was observe in MuLV-infected tumor cells as compared with noninfected cells when spleen cells from syngeneic tumor-bearing hosts (TBH) were used as a source of effector lymphocytes. The cytotoxicity of spleen cells against MuLV-infected tumor cells was specifically directed to the tumor-associated antigen (TAA), but not to the virus-associated antigen. However, there was no quantitative difference in the amount of TAA on the cell membranes between virus-infected and noninfected tumor cells as measured by a quantitative absorption test of anti-TAA serum. The cytotoxic activity of spleen cells from TBH against MuLV-infected tumor cells was abrogated by the treatment of anti-T-serum plus complement and significantly decreased after trypsin treatment. Spleen cells from normal rats given injections of immune sera from TBH acquired the cytotoxic activity against MuLV-infected tumor cells.