Interferons, cell glycoproteins synthesized in response to viral infections and various nonviral inducers, have proved therapeutically effective for viral infections in experimental models and in humans. Current evidence suggests interferons may also prove effective as antitumor agents in humans. Potent effects on cellular function result from interferons. Cell surface structure and enzyme levels are altered. Immunologic responses thought to be important in tumor immunity are augmented. Interferons have antiproliferative effects on the replication of normal and neoplastic cells. Interferons are effective in animals against tumors of both viral and nonviral origin. Clinical trials in cancer have been limited by the availability and cost of human interferons. However, results in small numbers of patients have been encouraging. This paper review experimental and clinical findings regarding the rationale for use of interferons in neoplastic disease.