TNF-alpha enhances HIV-1 replication in acutely and chronically infected cells and likely contributes to the wasting associated with the acquired immunodeficiency syndrome. Agents that inhibit TNF-alpha activity should theoretically delay the progression of disease, and several are currently in clinical trials. We hypothesized that IL-10, a cytokine that suppresses the gene expression and synthesis of TNF-alpha in monocytic cells, might inhibit HIV-1 replication. As expected, IL-10 suppressed PMA-induced TNF-alpha production in U1 cells; however, when U1 cells were cultured in the presence of PMA and increasing doses of IL-10, a dose-dependent increase in HIV-1 expression was observed. IL-10 also enhanced IL-1 beta-, TNF-alpha-, and GM-CSF-induced HIV-1 expression in U1 cells, and this occurred, at least in part, at the level of transcription. We next stimulated cells under conditions of TNF-alpha blockade. When PMA-induced TNF-alpha activity and HIV-1 replication were blocked by the presence of soluble TNF receptors, IL-10 independently enhanced HIV-1 replication. In contrast, other agents that are capable of blocking TNF-alpha synthesis or TNF-alpha activity either had no effect (IL-13 and IL-4) or inhibited HIV-1 expression (soluble TNF receptors and pentoxifylline) in U1 cells. These data suggest that IL-10, while inhibiting TNF-alpha synthesis, has an independent mechanism of action that enhances HIV-1 replication. Therefore, IL-10 may have undesirable effects in HIV-1-infected patients.