Interleukin-2 (IL-2) is a 15,000 dalton glycoprotein produced naturally by human T-cells during an immune response. IL-2 has been demonstrated to have substantial activity alone or in combination with the adoptive transfer of lymphokine-activated killer cells in murine tumor models. IL-2 derived from both natural (Jurkat human T-cell tumor) and recombinant (Escherichia coli) sources has been studied in Phase I protocols designed to evaluate toxicity in patients with a variety of solid tumors and to ascertain improvement in clinical parameters and immunologic status. A total of 16 patients (7 with acquired immune deficiency syndrome [AIDS] and 9 with non-AIDS malignancies) were treated with Jurkat derived IL-2. The total maximum dose (1.3 X 10(5) U/kg) was limited only by supply of this reagent. A total of 25 patients have been treated with recombinant IL-2 (RIL-2) alone. Dose-limiting toxicity manifested by marked malaise and weight gain was achieved with doses of RIL-2 of 10(6) U/kg as a single bolus or 3000 U/kg/hr. IL-2 could be administered intraperitoneally with similar toxicity. Minimal renal or hepatic toxicity was demonstrated. Hematologic toxicity was limited to mild anemia (25/25), thrombocytopenia (10/25), and marked reversible eosinophilia (15/25). Pronounced weight gain greater than 2 kg (16/25) occurred in most patients, primarily those who received cumulative doses of greater than 1-3 X 10(5) U/kg of IL-2. The weight gain amounted to as much as 10% to 20% of the pretreatment weight over 3 weeks of treatment and limited our ability to give higher doses. Two partial responses (greater than 50% decrease in cross sectional diameters) were seen in two patients with melanoma metastatic to the lung.