This study examined the role and source of endogenous interleukin-10 (IL) secretion in visceral leishmaniasis (VL). The amounts of endogenous and Leishmania specific IL-IO and interferon-gamma (IFN) secreted by peripheral blood mononuclear cells (PBMC)from VL patients were compared. The correlation coefficient between endogenous IL-10 secretion and Leishmania specific IFN-gamma was -0.77, sug Sestirzg a major role for endogenous IL-10 secretion in VL. The effects of CD4+ and CD8+ T cell clones, isolated from a treated VL patient, on IL-10 secretion were assayed by mixing the clones with autologous, inactivated PBMC . The CD8+ T clones mediated increased levels of lL-10 secretion in the presence of PBMC alone suggesting that CD8+ T cells may, mediate endogenous IL-10 secretion.