Recombinant gamma interferon (ry-IFN) was administered s. c. daily to 26 patients with advanced cancer. Patients were assigned to one of six doses: 0.5, 1, 2, 4, 6, or 8 million units (MU)/m 2 per d. The major toxicities were an influenza-like syndrome and fever, seen in all patients. Dose limiting toxicity occurred in 4 of 4 patients treated at 8 M U / m 2. One patient with nodular poorly differentiated lymphocytic lymphoma had a mixed response, and two patients with renal cell cancer have had stabilization of disease for > 10 and > 12 months. Pharmacokinetic analysis, by radioimmunoassay, revealed mean serum ry-IFN concentrations up to 17 ng/ml, with maximal serum levels noted 6 to 13 h after injection. In vivo immunomodulation was assessed by natural killer (NK) cytotoxicity, monocyte activation as determined by cell surface expression of HLA-Dr, and peripheral blood mononuclear cell phenotype analysis by flow cytometry. The m e a n T4/T 8 ratio increased from 2.1 pretreatment to 4.1 after 24 h of treatment, but returned to baseline after 7 and 28 days of treatment. Augmentation of NK function was noted after 7 days of treatment. Monocyte cell surface expression of HLA-Dr increased after 28 days of treatment at the three lowest doses. In conclusion, daily s. c. ry-IFN can be easily administered on an outpatient basis with minimal local skin toxicity, results in prolonged serum levels, and is associated with immunological changes of potential antitumor significance. Further study of the in vivo immunomodulatory effects induced by ry-IFN is indicated to help define the optimal treatment regimen.