Lymphokine-activated killer (LAK) cells can be generated by incubating peripheral blood lymphocytes in recombinant Interleukin―2. LAK cells kill fresh autologous and allogeneic human tumor cells in vitro. The adoptive transfer of these LAK cells into tumor-bearing hosts can mediate the cure of disseminated cancer in a variety of animal model systems. But the mechanism of target cell killing by LAK cells is as yet undefined. We have postulated that such killing may involve some soluble cytotoxic factors produced and secreted by LAK cells. LAK cells were induced by mitogens and tumor cells to secrete cytotoxic factors against L929 cells. LAK cells produced cytotoxic factors within 2 hr after induction and maximal production was observed 6 hr after induction. There was a good correlation between in vitro LAK cell activity and production of cytotoxic factors. We have identified one of these cytotoxic factors as tumor necrosis factor (TNF). LAK cells lost their in vitro cytotoxic activity and TNF producibility after treatment with OKT-8 antibody. These findings suggest that TNF may be a mediator of in vitro LAK cell activity.