HIV-1 infection of promonocytic U937 cells was used to examined induction of IFN-alpha/beta gene expression and to determine the inhibitory effects of IFN-alpha 2 and/or AZT on de novo HIV-1 infection, initiated either by coculture of virus-shedding U9-IIIB cells with uninfected cells or by incubation of U937 cells with virus-containing supernatants. Usually 21-28 days were required to transmit virus to greater than 90% of the cell culture. HIV-1 infection did not stimulate constitutive production of endogenous IFN-alpha 1/alpha 2 or IFN-beta genes, although induction of IFN RNA was observed following coinfection with the paramyxovirus Sendai. Exogenous rIFN-alpha 2 treatment decreased the intracellular accumulation of viral RNA 5- to 20-fold as determined by Northern blot analysis and the extracellular levels of HIV-1 as measured by p24 ELISA antigen capture. The effect was most dramatic at a time coincident with the rapid increase in virus spread through the cell culture (Days 10-18). During the fourth week of infection, HIV-1 multiplication was able to overcome the IFN-induced block in virus spread. AZT was not effective in limiting virus spread in the cocultivation experiments. When U937 cells were infected with virus-containing supernatants from U9-IIIB cells, IFN and AZT acted in combination to limit the number of infected cells and to inhibit intracellular accumulation of viral RNA. These experiments suggest that subtle differences in the mode of virus transmission in monocytic cells may alter the efficacy of combination antiretroviral therapy.