Killer helper factor (KHF) was previously found to be produced by a human T cell hybridoma, 24A . CA2. We studied the therapeutic effects of interleukin-2 (IL-2) and KHF on the inhibition of pulmonary metastases of syngeneic Lewis lung carcinoma (3LL) in C57BL/6N mice. Multiple subcutaneous (sc) injections of IL-2 plus KHF had significantly more effect than injections of IL-2 alone in inhibiting spontaneous pulmonary metastases and prolonging survival of the mice. The effect of KHF with IL-2 on induction of lymphokine (IL-2)-activated killer (LAK) activity against P-29 cells was examined in the murine system. Spleen cells generated LAK activity after incubation for 4 days with more than 500 U/ml of IL-2. In contrast, KHF alone did not render spleen cells cytotoxic. The combination of these lymphokines at subthreshold concentrations, however, resulted in significant in vitro induction of LAK activity. The LAK activity of splenocytes incubated with IL-2 plus KHF was maximal after 4 days, and persisted for longer than that of cells treated with IL-2 alone. The LAK cells induced by KHF plus IL-2 were also cytotoxic to FBL and YAC-1 cells. Moreover, spleen cells of mice bearing lung metastases could be induced to the cytotoxic state by sc injections of IL-2 plus KHF. These results indicate that combination treatment with IL-2 and the new lymphokine KHF should be useful clinically in inducing LAK activity for inhibition of pulmonary metastases.