We report a clinical study of the pharmacokinetics, toxicity, and biological activity of i.v.- and i.m.-administered recombinant gamma-interferon (rIFN-gamma) consisting of 143 amino acids. Ten patients with metastatic cancer were given rIFN-gamma at doses of 0.01 to 2.5 mg/sq m by alternating i.m. and i.v. bolus injections with a minimum intervening period of 72 h. After i.v. administration, rIFN-gamma was cleared monoexponentially with a short half-life of 25 to 35 min as determined by bioassay and enzyme immunoassay. After i.m. injection, a longer half-life of 227 to 462 min was measured by enzyme immunoassay. Serum titers were detected by bioassay only at high doses, suggesting partial loss of antiviral activity at the i.m. site. However, other biological effects were retained as evidenced by fever, chills, and fatigue after both routes of administration and granulocytopenia after i.m., but not i.v., doses. Two of ten patients showed objective evidence of tumor regression. These data suggest that further studies with i.m. as well as prolonged i.v. infusions of rIFN-gamma are indicated.