The functional activity of Moloney murine sarcoma virus (M-MSV)-specific T lymphocytes in vivo was assayed by the i.v. injection of virus-specific T lymphocytes into T cell-deficient "B mice". Virus-specific T lymphocytes generated in mixed lymphocyte tumor cell cultures were transferred i.v. into syngeneic "B mice" injected simultaneously at a distant site with the virus. These experiments indicated that a low dose (1 X 10(6) cultured cells) of infused lymphocytes can afford protection. To define the T lymphocyte subpopulation which was active, Lyt-2+ lymphocytes were selected by "panning" on plastic petri dishes coated with anti-Lyt-2 monoclonal antibody, and Lyt-2- lymphocytes selected by treatment with anti-Lyt-2 monoclonal antibody and complement. The results indicated that a Lyt-2+ lymphocyte-enriched population was more efficient in conferring protection against M-MSV-induced tumors. To investigate if cytolytic T lymphocytes (CTL) alone had a protective effect, a M-MSV-specific CTL clone was transferred in the same model system. The results demonstrated that a M-MSV-specific CTL clone prevented M-MSV-induced tumor growth and also induced the destruction of syngeneic Moloney murine leukemia virus (M-MuLV)-induced MBL-2 leukemic cells in the peritoneal cavity. However, the cell dose required to obtain protection using a CTL clone was higher than that which was effective when mixed lymphocyte tumor cell culture cells were used. To assess the ability of the transferred cells to home and to repopulate the lymphoid organs of the "B mice", the frequency of virus-specific CTL precursors in the spleen was evaluated by limiting dilution analysis. The results indicated that lymphocytes from mixed lymphocyte tumor cell cultures can be recovered from the spleens of "B mice" injected i.v. 25 days earlier. On the contrary, following the transfer of an active CTL clone, a very low frequency (less than 1/200,000 cells) of virus-specific CTL precursors was present in the spleens of recipient animals. The same M-MSV-specific CTL clone did not yield protection against M-MSV-induced tumors or MBL-2 leukemic cells when injected i.v. into M-MuLV tolerant mice.