We report a clinical study of toxicity and pharmacokinetics of intravenously administered natural interferon-gamma. Twenty three cases with metastatic cancer were given interferon-gamma at doses of 10 X 10(4)-400 X 10(4) IU in single injection. Another twenty three cases were administered at doses of 5 X 10(4)-50 X 10(4) IU/day every day for thirty days. Side effects associated with natural interferon-gamma administration were qualitatively similar to those previously reported for alpha, and beta interferon treatment. After intravenous injection, interferon-gamma was cleared monoexponentially with a short half-life of 120 minutes from the circulation. One case with disseminated thyroid cancer showed minor response.