alpha-Interferon (IFN alpha) blocks replication of human immunodeficiency virus (HIV)-1 in vitro by interfering with the release of mature virions. Clinical trials have addressed the in vivo effects of IFN alpha, both alone and in combination with other agents, in a variety of patients at all stages of HIV-1 infection. Patients with late stages of HIV-1 infection (CD4 counts under 100) show few positive results following treatment with IFN alpha. Patients with earlier stages of HIV infection, however, may benefit from treatment with this agent. Several clinical trials have demonstrated the activity of interferon in the treatment of patients with acquired immunodeficiency syndrome, Kaposi's sarcoma, and CD4 counts over 200. In these trials, response rates of approximately 40% have been reported, with the probability of response directly correlated with the level of CD4 cells. These antitumor effects have been associated with declines in the circulating levels of the HIV-1 core antigen p24. alpha-Interferon activity has also been studied in patients concomitantly receiving zidovudine. In these studies, neutropenia, reversible with the concomitant administration of granulocyte macrophage colony-stimulating factor, has been the most common dose-limiting toxicity. Both the antitumor and antiviral activities of combination therapy appear to be at least as good as those observed when single agents are used. Controlled clinical trials are currently under way to evaluate the role of interferon therapy, both alone and in combination with zidovudine, in patients with early HIV infection.