Human interleukin-10 (IL-10) is a poorly characterized cytokine that is capable of inhibiting interferon-gamma (IFNg) and granulocyte-macrophage colony stimulating factor production by mitogen-activated human peripheral blood mononuclear cells (PBMC). Human IL-10 has the same biological effects as murine IL-10 on murine cell targets. These effects include stimulating the proliferation of murine mast cells, increasing survival and expression of MHC Class II antigens on murine B cells, inducing the growth of peripheral T cells, and serving as a cytotoxic differentiation factor. With respect to the present study, the most relevant activities of human IL-10 are its abilities to inhibit IFNg production and T cell proliferation in response to mitogens. What role, if any, that IL-10 has in the course of HIV-1 infection is unknown. It is possible that the suppressive activity of IL-10 upon IFNg production may result in enhancement of HIV replication given the antiviral properties of IFNg. Previous work has shown that H9 cells demonstrate evidence of productive HIV-1 infection seven days post- infection using p24 levels as an indicator of infection. The addition of IL-10 to such cultures at varying doses and times post infection should determine whether IL-10 has an effect on viral replication. Additional proliferation studies would indicate whether the H9 cell line is subject to the anti-proliferative properties of IL-10. Further studies using experimentally infected human peripheral blood lymphocytes may provide information on the role of this cytokine in the suppression of IFNg production, T cell proliferation, and the enhancement of viral replication in a test system in which the presence of other cytokines such as IL-2 and IL-4 may interact with IL-10.