The activation of human peripheral blood leukocytes or murine splenocytes with interleukin-2 (IL-2) generated cells that were lytic in vitro for a variety of fresh tumor cells. The adoptive transfer of such lymphokine-activated killer (LAK) cells to mice with established pulmonary sarcoma metastases was highly effective in reducing the number (and size) of these tumor nodules when combined with repeated injections of recombinant IL-2. These findings provide a rationale for clinical trials of the infusion of human LAK cells generated with recombinant IL-2 as well as Phase I trials of the infusion of recombinant IL-2 systemically into humans.