A total of 11 patients were treated on an escalating, single dose trial of recombinant gamma interferon (rIFN-gamma), 6 patients by the i.m. and 5 patients by the i.v. route of administration. Dose ranges within each individual were from 0.05 mg/m2 of IFN (1 mg greater than or equal to 10 X 10(6) units of IFN) escalating to 10 mg/m2. All dosages were delivered twice weekly and the i.v. dose was infused over 5 min. The most common toxicities encountered included fever, chills, fatigue, anorexia, and granulocytopenia. The influenza-like symptoms were very similar to those encountered with IFN-alpha but were generally less severe. The granulocytopenia was dose-related and transient with recovery generally seen within 48-72 h following administration of rIFN-gamma. Absolute granulocyte counts only rarely dropped below 1000 mm3. Hepatotoxicity was not observed. IFN levels were determined by both a bioassay and an enzyme-linked immunosorbent assay. By the i.v. route, the peak level of IFN activity could usually be seen at completion of the infusion with a serum half-life of 30 min. By the i.m. route, the peak level of serum activity was generally detected between 4-8 h with a serum half-life of 4.5 h after the initial elimination phase. Peak IFN levels appeared to correlate with maximum toxicity. One patient with melanoma had a 25% reduction in a cutaneous lesion, but there were no other minimal, partial, or complete responses.