A phase I trial was performed with a new interleukin-2 (IL-2) given as a continuous intravenous infusion in patients with solid tumors. The objectives of the study were to examine the feasibility of administering IL-2 in 4-day cycles for 4 consecutive weeks, and to investigate the response pattern of peripheral blood lymphocytes. Tumor necrosis factor (TNF) and IL-2 serum concentrations were also measured. Prior to this study, IL-2 had been tested at increasing dosages during one 4-day cycle, and it appeared that a dose of 1300 mcg/m2/day was tolerated. However, when this treatment schedule was maintained for 4 consecutive weeks, the maximum tolerated dose was 430 mcg/m2/day. In this schedule, a dose-dependent progressive increase in rebound lymphocyte count occurred after each weekly cycle, resulting in a 5-70-fold increase after the 4th cycle. Serum TNF peak concentrations also showed a tendency to increase during each subsequent cycle, while serum IL-2 peak concentrations showed a paradoxical decrease. Clinical toxicity comprised several events, which, possibly, could be ascribed to autoimmune phenomena. Myocardial infarction as a late toxicity of IL-2 is suggested. One complete response (renal carcinoma) and two partial responses (renal and breast carcinoma) were documented, one of these occurring in a patient who previously had shown a transient response on interferon therapy.