IFN-alpha is in plasma of HIV-1 infected patients during early and late-stage disease and may play a role in control of virus replication. The stimulus for IFN-alpha production, the cells that produce this cytokine, and the effectiveness of this IFN-alpha response for control of virus infection are not yet defined. Culture fluids from freshly isolated PBMC of HIV-1 seronegative donors contained high levels of IFN-alpha after exposure to 100 to 1000 infectious HIV-1 particles per culture. Levels of IFN-alpha induced by HIV-1 were directly dependent on the number of monocytes in cell preparations: No IFN-alpha was detected from T cell-enriched PBMC. In monocyte cultures, induction of IFN-alpha by HIV-1 was relatively specific: Levels of IL-1 beta, IL-6, IFN-gamma, and TNF-alpha remained at baseline. Capacity of HIV-1 virions to induce IFN-alpha was not dependent on virus replication. IFN-alpha was induced by (a) heat-inactivated HIV-1, (b) virions from 8E5 cells, a cell line that releases noninfectious HIV-1, (c) HIV-1-infected cells fixed in paraformaldehyde, and (d) T cell-tropic HIV-1 that binds to but does not infect monocytes. Capacity of HIV-1 virions and HIV-1 infected cells to induce IFN-alpha was completely inhibited by soluble rCD4 or mAb against CD4 or gp120. Antibodies against CD4, however, did not induce monocytes to produce IFN-alpha. HIV-1-induced IFN-alpha production was inhibited by antibodies against both V3 loop determinants and the CD4 binding site of gp120. Further, sera and purified immunoglobulin from HIV-1 infected patients also inhibited HIV-1-induced IFN-alpha production. These observations suggest that potentially protective antiviral responses associated with IFN-alpha production in HIV-1 infected patients are inhibited by the development of antibodies against gp120.