We performed an escalating dose study of the combined administration of interleukin-2 (IL-2) and alpha-interferon (alpha-IFN) in 94 patients with metastatic cancer. Patients received alpha-IFN at a dose of 3 x 10(6) U/m2 in conjunction with IL-2 at doses of either 1 x 10(6) U/m2 (six patients), 3 x 10(6) U/m2 (32 patients), or 4.5 x 10(6) U/m2 (26 patients). Thirty patients received alpha-IFN at 6 x 10(6) U/m2 plus IL-2 at 4.5 x 10(6) U/m2. Patients each received cytokine as an intravenous bolus infusion every 8 hours for up to 5 consecutive days and after a 10-day rest received a second cycle of combination cytokines. Of the 91 patients evaluable for response, seven patients had a complete regression of cancer, and 18 had a partial regression. At the four increasing dose levels used in patients with renal cell cancer (35 patients) or melanoma (39 patients), objective responses were seen in 17% (of six patients), 24% (of 25 patients), 38% (of 16 patients), and 41% (of 27 patients), respectively. Of the 25 total responding patients, 16 are still responding 5 to 14 months after treatment. The toxicities associated with the combined administration of IL-2 and alpha-IFN were similar to those expected from each agent alone. There was one treatment-related death in the 94 patients treated in this study. Thus, using increasing doses of the combination of IL-2 and alpha-IFN, it appears that response rates may be related to the doses of the cytokines used, and that at the highest doses of these combination cytokines, response rates may be higher than those for either cytokine alone. A prospective randomized trial comparing the cytokine combinations with each cytokine administered alone is necessary as is the extension of this combination cytokine treatment to patients with other types of solid cancer.