Recombinant interleukin 4 (IL-4) stimulated extracellular (EC) and intracellular (IC) production of human immunodeficiency virus (HIV) from infected human blood-derived monocytes and macrophages when incubated with the cells after but not before virus inoculation. Significant stimulation was observed in 20 of 27 experiments with monocytes (inoculated with HIV immediately after adherence) and 10 of 13 experiments with macrophages (inoculated after 5 days adherence) using a total of 30 normal donors of monocytes and macrophages, and 11 recent isolates of monocytotropic HIV strains (after one passage in mononuclear cells). Marked increases in EC and IC HIV antigen were observed in some experiments, which were comparable with the maximal stimulatory effects of other cytokines such as IL-2. IL-4 also had similar effects on infectious HIV concentration as measured by reverse transcriptase and TCID50 assays. Antibody to IL-4 prevented the stimulatory effect of the cytokine. The proportion of monocytes and macrophages infected by HIV, as determined by in situ hybridization, also increased after incubation with IL-4 for 7 days. The most marked effects were observed with HIV-infected macrophages, for which the proportion of unstimulated infected cells was lower (35 to 45% increasing to 66 to 70% with IL-4 treatment). There was also an increased proportion of cells with high granule concentrations, suggesting that IL-4 increases the intracellular concentration of viral nucleic acids. This was supported by semi-quantitative hybridization experiments showing that total HIV RNA increased in IL-4-stimulated monocytes 48 to 96 h after HIV inoculation. A marked increase in aggregates was observed on day 7 in HIV-infected monocytes treated with IL-4, compared to that in HIV-infected cells alone or IL-4-treated uninfected monocytes. These findings suggest that IL-4 stimulates HIV replication in the early phases of infection and may also facilitate virus transmission by aggregate formation.