The interferons are an important first member of a family of biologic response-modifiers used in treating human malignancies. Activities associated with the interferons include inhibition of viral replication, influence on cellular protein production, direct antiproliferative effects, and a variety of modulatory effects on the immune response. These regulatory functions of interferon underlie the interest in its use as an anticancer agent. Alpha interferon is the most extensively studied interferon species. Although antitumor activity has been seen both in vitro and in vivo in some solid malignancies, the most impressive responses have occurred in the hematologic malignancies. More than 90 percent of patients with hairy cell leukemia have a sustained recovery of their peripheral blood cell counts with alpha interferon therapy. Approximately 50 percent of patients with low-grade non-Hodgkin's lymphoma and cutaneous T cell lymphoma demonstrate a response to alpha interferon. More than 80 percent of patients with chronic myelogenous leukemia have a response to alpha interferon, and in one study, nearly half of the patients with response had complete suppression of the Philadelphia chromosome clone on at least one examination. Ongoing clinical trials are addressing such issues as optimal dosage, duration of alpha interferon therapy, and combinations of alpha interferon with other biologic agents, chemotherapy drugs, and radiation.