The antiviral and antitumor mechanisms of action, pharmacokinetics, dosage considerations, and adverse effects of interferon are reviewed, and the results from clinical studies are summarized. Interferons are proteins that can induce a nonspecific resistance to viral infection by several mechanisms, including the inhibition of protein synthesis, inactivation of viral RNA, and enhancement of phagocytic and cytotoxic mechanisms. There are three classifications of interferons on the basis of their antigenic specificities--alpha (alpha), beta (beta), and gamma (gamma) interferon. Interferon is not absorbed from the gastrointestinal tract because it is a protein susceptible to proteolytic digestive enzymes. However, the absorption of alpha-interferon by the intramuscular and subcutaneous routes is nearly complete; peak serum concentrations occur at approximately two hours and four hours following i.m. and s.c. injections, respectively. The usual method of administration is by intramuscular injection, and the doses range from 1 to 10 million units per injection. Since blood concentrations decrease sharply within 24 hours, interferon is usually given on a daily basis, and the duration of therapy varies according to patient response. Adverse effects from interferon are generally transient and reversible. Some success has been reported with the use of interferon in the treatment of chronic hepatitis B infection. Acute fulminant hepatitis infection has also been treated with interferon. Patients with cytomegalovirus infection treated with interferon showed a decrease in the urinary virus but no important clinical benefits. The treatment of varicella-zoster (chicken pox) virus has been successful. The use of interferon by injection to treat influenza and the common cold seems unlikely because of its short duration of action and the self-limited nature of these infections. Varying success has been obtained when malignant tumors have been treated with interferon. Many types of viral infections and tumors have been treated with interferon with varying results. More research must be conducted before a final analysis of the clinical value of interferon can be made.