The interferons are the best known of biologic antineoplastic agents. Progress with the clinical application of interferons to cancer has been slow and complicated by the need for attention to a new spectrum of therapeutic and toxic effects manifest by the interferons. This summary of current phase I and II trial results with the interferons establishes their clinical potential. The maximally tolerated dosages of the most common species of interferon alpha produced in eukaryotic cells as well as by recombinant DNA technology in bacteria are now described in a variety of different disease states. "Naturally" produced eukaryotic as well as bacterially synthesized interferons have a similar, wide range of biologic effects in vitro and in vivo. Antiviral, antiproliferative, immunologic, and enzymologic functions of the interferons relevant to antineoplastic functions are under study. Knowledge of these mechanisms should improve the clinical results obtained in human cancer. Species and subspecies differences in the activity of interferons may lead to selective use of the pure interferon subspecies, alone or in combination. The use of the interferons and other antineoplastic biologics, such as antibody or chemotherapy, are subsequent goals that are now on the horizon.