We now have a basis for a more rational approach to rapid evaluation and development of antiviral drugs by screening for activity in vitro, testing for toxicity and efficacy in animals, and clinical testing in humans. Acyclovir is a prototype of this improved process. Interferon has a beneficial effect against CMV infection in renal transplant patients and has promising results in the treatment of papillomas and rhinovirus infections. It does not seem to be as effective against genital herpes or varicella zoster as acyclovir. Ribavirin is effective against respiratory syncytial virus infections and Lassa fever. Varicella-zoster virus is highly sensitive to bromovinyl deoxyuridine in vitro. Phosphonoformate is effective in herpes simplex in animals but of little clinical benefit topically in human recurrent A2 herpes. Zidovudine may decrease mortality rates and infectious complications in patients with acquired immunodeficiency syndrome. DHPG (9-(1,3-dihydroxy-2-propoxymethyl]guanine is useful in treatment of cytomegalovirus and infection in immunocompromised patients. The prodrug of acyclovir results in high blood levels of acyclovir and shows promise in the treatment of varicella-zoster infections. Many halogenated pyrimidine nucleoside analogs are being developed. Buciclovir is another acyclic guanosine analog effective against herpes simplex virus in vitro. 2'-nor-cyclic guanosine monophosphate has a broad antiviral spectrum of action. Interleukin-2 is being investigated. Combined therapies of two or more antiviral drugs or antiviral drugs and other treatments are being studied.