Forty-seven patients with metastatic malignant melanoma were treated with two 5-day cycles of 100,000 U/kg recombinant interleukin-2 (IL-2) intravenously (IV) every 4 hours separated by 1 week. This dose and schedule of IL-2 were identical to those used in a previous combined IL-2 and lymphokine-activated killer (LAK) cell phase II clinical trial of the IL-2/LAK Working Group. Patient eligibility criteria, and clinical management guidelines were similar to those used in the previous trial. Forty-six patients were assessable for response. Objective responses were observed in 10 of 46 patients (two complete responses [CRs], eight partial responses [PRs]) or 22% with responses occurring in lung and liver as well as lymph nodes and subcutaneous sites. The median response duration was 8 months. Toxicity was significant; three patients developed myocardial infarction, and one patient died during therapy. Overall the toxicity and response rate for single-agent IL-2 are similar to that observed with IL-2 administered in combination with LAK cells in the previous trial. These results suggest that single-agent therapy with IL-2 when administered in this schedule has significant antimelanoma activity in humans, and that LAK cells generated from peripheral blood add little to the antimelanoma activity of this dose and schedule of IL-2.