The production of tumor-specific cell-mediated cytotoxicity following in vitro sensitization of C57BL spleen cells against a syngeneic 3LL Lewis lung carcinoma was studied. Lymphocytes were sensitized on monolayers of the tumor cells for 4-5 days. The cytotoxicity was assayed by measuring the reduction in 3H-leucine and 3H-thymidine incorporation by target cells after interaction with the sensitized lymphocytes. Spleen lymphocytes sensitized on monolayers of 3LL tumor cells caused a high extent of lysis; such cells tested on C57BL or C3H fibroblast targets evoked only a low level of cytotoxicity. C57BL spleen cells sensitized on C57BL fibroblasts caused a low level of cytotoxicity when tested on a 3LL target. Thus cytotoxicity appeared to be tumor specific. The reduced incorporation into protein and DNA of target tumor cells caused by the sensitized lymphocytes was a measure of cell injury, which was more sensitive than direct cell count or uptake of 51CR. Lymphocytes from syngeneic tumor-bearing mice, tested 13-25 days after tumor inoculation, did not manifest in vitro cytotoxicity. On the contrary, such lymphocytes sometimes appeared to have a promoting effect on the tumor cells.