Interleukin-2 (IL-2) and the adoptive transfer of lymphokine-activated killer (LAK) cells have been used to reduce tumor mass in intraperitoneal, pulmonary, and hepatic models. Unfortunately, repeated cycles of IL-2 plus LAK cells have not been demonstrated to exert additive antitumor effects. We investigated the responses that repeated challenge with LAK cells and IL-2 provoked in normal BL/6 mice. Six immunizations of LAK cells generated in media containing fetal calf serum abrogated the ability of mice to respond to IL-2 plus LAK immunotherapy in a peritoneal carcinomatosis model. Serum from these treated mice would cause lysis of LAK cells or cultured lymphocytes in the presence of complement. LAK cells generated in the presence of normal mouse serum were not lysed. Similarly, the antitumor effects of IL-2 plus LAK immunotherapy were abrogated in mice pretreated with six courses of IL-2. We found that LAK cells cytolysis could not be generated if serum from IL-2-pretreated mice was present in vitro with splenocytes and IL-2. These data suggest that antibody against absorbed antigens on LAK cells and serum inhibitors of IL-2 are generated following repeated challenge. These observations may be important in interpreting IL-2 plus LAK immunotherapy studies in murine models and may modify our attempts to treat cancer patients.