Combinations of interferon-alpha and interferon-gamma demonstrate synergistic antiviral and anti-proliferative activity in vitro. Therefore, we initiated a clinical study of combination interferon therapy in humans. Eighteen patients with metastatic solid tumors received daily intramuscular (IM) injections of recombinant interferon-alpha-A (IFN alfa-2a, Roferon-A; Hoffman-LaRoche, Nutley, NJ) and recombinant IFN-gamma (rIFN-gamma) for 6 weeks. The dose levels were 0.5, 1.0, 2.0, and 5.0 X 10(6) U/m2/d of each interferon. A minimum of two patients were entered sequentially at each dose level. Fever, chills, fatigue, and a greater than or equal to 50% drop in granulocyte counts were observed at all doses. Severity of symptoms corresponded to increasing dose levels. In contrast to the tachyphylaxis to these symptoms that usually develops in patients treated with the individual interferons, many patients on this study experienced persistent fever and worsening fatigue over 6 weeks. The maximum tolerated dose was 1 X 10(6) U/m2/d of each interferon. One patient with renal-cell carcinoma achieved a partial remission (duration, 3 months). Enzyme-linked immunoassay analysis in all four patients for whom complete data were available revealed that peak blood levels of IFN alfa-2a on day 22 were about tenfold higher than on day 1. Because of the possibility of cumulative toxicity, the recommended starting dose for further studies is 0.5 X 10(6) U/m2/d of each interferon, with escalation to 1.0 X 10(6) U/m2/d after 1 month if tolerance is acceptable. Phase II investigations to explore the antitumor efficacy of this regimen are planned.