Based upon in vitro and in vivo synergistic activity of Type I and Type II interferons (IFNs) in preclinical in vitro and in vivo studies, we initiated a phase I trial evaluating the doses, safety, and pharmacokinetics of combinations of recombinant DNA-produced human IFN-beta ser and IFN-gamma in 27 patients with cancer. Twenty-four patients were treated with a 2-hour infusion of IFN-gamma, followed by a 10-minute iv injection of IFN-beta ser, three times a week. Patients were entered on fixed dose levels of 1 X 10(6), 3 X 10(6), 10 X 10(6), 30 X 10(6), and 100 X 10(6) units of each IFN. In addition, three patients were treated at the highest dose level with a 10-minute iv infusion of IFN-gamma and a 10-minute iv infusion of IFN-beta ser. The maximally tolerated dose when administered by this schedule for greater than or equal to 4 weeks was 30 X 10(6) units of each IFN. Dose-limiting side effects at doses of 100 X 10(6) units of each IFN consisted of fatigue, nausea, vomiting, anorexia, paralytic ileus, and neutropenia. The most common side effects at the three highest dose levels were fever, rigors often requiring parenteral meperidine, and constitutional symptoms. Reversible elevations in SGOT and LDH were also noted. Serum IFN levels were dose related, with peak titers occurring immediately after IFN administration. One patient with a nodular mixed lymphoma had a partial response which has been sustained for over 1 year. We conclude that combinations of IFN-beta ser and IFN-gamma can be safely administered on a chronic basis without enhanced or cumulative toxic effects.