Twenty-six patients with metastatic cancer were entered into a phase I trial of concurrent recombinant interleukin-2 (IL-2) and recombinant interferon-gamma (IFN-gamma). IL-2 was administered as a continuous intravenous infusion for 5 days. IFN-gamma was administered by a daily intramuscular (IM) injection during the 5 days of IL-2 administration. Treatment was repeated twice after 9-day rest periods. After a 2-week rest, patients without evidence of tumor progression were retreated. Natural killer (NK)- and lymphokine-activated killer (LAK)-cell activity were assayed in each patient before treatment, on day 1, and on day 5 of each cycle. Constitutional symptoms occurred in most patients but were not dose-limiting. Other toxicities included hypotension responsive to fluids, transient elevations in liver function tests, erythema/pruritus, eosinophilia, and transient leukopenia/thrombocytopenia. The maximum-tolerated dose (MTD) of the combination was 1 x 10(6) U/m2/d of IL-2 combined with 0.50 mg/m2/d of IFN-gamma. The dose-limiting toxicity was pulmonary manifesting as rales and shortness of breath. The dose of the combination that resulted in the optimal generation of in vivo LAK-cell activity was a dose of at least 0.25 mg/m2/d of IFN-gamma combined with 1 x 10(6) U/m2/d of IL-2. Objective clinical responses were seen in five of 26 patients. These included a partial response of 2 months duration in a patient with non-Hodgkin's lymphoma (NHL), mixed responses in a patient with NHL and two patients with renal cell carcinoma (RCC), and an ongoing assessable response in a patient with bone metastases from RCC. The recommended dose for phase II trials of this combination is 0.50 mg/m2 of IFN-gamma and 1 x 10(6) U of IL-2.