Gamma-interferon appears to be a pivotal molecule in the immune system. Recombinant DNA technology has permitted the cloning and expression of the gene for gamma-interferon (gamma-IFN), leading to an exponential increase in the level of knowledge both in vitro and in vivo. Laboratory evidence suggests a clinical role for gamma-IFN in collagen vascular disease, chronic granulomatous infectious diseases, hematology, and medical oncology. Phase I trials have identified the toxicities; these toxicities are primarily clinical and include fever, fatigue, flu-like symptoms, and hypotension. Antitumor activity has been noted in the early development of the drug.