Optimal conditions for the in vitro production of cytotoxic lymphocytes against autologous lung cancer cells were studied. In the time-course experiments, fresh lymphocytes did not exhibit any cytotoxicity against autologous tumor cells, but, when cultured in the presence of T cell growth factor (IL2), the lymphocytes became cytotoxic against autologous tumor cells 3 days after the initiation of incubation and the cytotoxicity continued to increase, reaching a maximum at day 7. The study, conducted to compare the effects of IL2 and phytohemagglutinin (PHA), demonstrated that although lymphocytes cultured with PHA did exhibit a significant cytotoxicity, lymphocytes cultured with IL2 showed much higher activity. Peripheral blood, regional lymph node (RNL) and distal lymph node lymphocytes, and tumor infiltrating lymphocytes were tested as sources for the production of cytotoxic lymphocytes. Among these 4 sources, RNL proved the most consistently reliable source of cytotoxic lymphocytes. The results of in vitro stimulation by autologous tumor cells were ambivalent. Twenty-five experiments that compared in vitro stimulation by tumor cells and no stimulation revealed that 9 cases (36%) showed enhancement of cytotoxicity after stimulation with tumor cells, 5 cases showed inhibition, and the remainder no effect. Characterization of lymphocytes by using monoclonal antibodies indicated that cells lytic for autologous tumor cells are OKT3+, OKT8+, OKT4-, OKM1-.