The adoptive transfer of lymphokine-activated killer (LAK) cells combined with low dose interleukin 2 (IL-2) mediates the regression of established pulmonary metastases in mice and has efficacy in the treatment of human cancer. Systemic administration of high dose IL-2 alone can mediate tumor regression. Cortisone acetate (CA), 25-75 mg/kg, was administered daily to mice receiving high dose IL-2 for 10 days. CA significantly reduced the toxicity induced by IL-2; 38 of 48 mice receiving CA survived compared to 0 of 30 controls (P less than 0.0001). In addition, CA administration caused a decrease in IL-2-induced 125I-labeled albumin leakage in mouse organs. However, CA abrogated the in vivo antitumor effect of high dose IL-2, and to a lesser extent the therapeutic effect of exogenous LAK cells plus lower dose IL-2. Mice treated with 100,000 units of IL-2 showed 98, 63, and 33% reductions of pulmonary metastases in Hanks' balanced salt solution, 25 mg Ca/kg, and 75 mg Ca/kg groups, respectively; treatment with LAK and 7,500 units of IL-2 resulted in reductions of 94, 77, and 57% in these same groups. CA treatment of animals did not affect LAK generation, although the absolute number of LAK precursors was greatly reduced. These results show that although CA can reduce the toxic effect(s) of IL-2, it can be detrimental to successful immunotherapy using this approach.