Interleukin-2 (IL-2) and gamma interferon (gamma-IFN) may be synergistic in inducing cell-mediated antitumor cytotoxicity. In order to determine the dose-limiting toxicities and define a maximum tolerated dose of these two agents in combination, we performed a Phase I clinical trial of intravenous IL-2 plus intramuscular gamma-IFN. Patients received both agents on a thrice-weekly schedule consisting of 4 weeks of treatment followed by 2 weeks of rest. Twenty-five patients were treated and received gamma-IFN doses between 0.05-0.25 mg/m2 (1-4 x 10(6) U/m2) with IL-2 doses from 0.33 mg/m2 to 2.33 mg/m2 (6-42 x 10(6) IU/m2). Two patients had partial responses of melanoma and adenocarcinoma of the lung lasting greater than 11 and 8 months, respectively. The toxicities of the combination were those expected from each agent, with no unusual effects, no irreversible organ toxicities, and no patient deaths. The doses recommended for outpatient administration on this schedule are IL-2, 2.0 mg/m2 plus gamma-IFN, 0.25 mg/m2, a dose combination that is unassociated with significant organ toxicity.