IL-10 is a regulatory cytokine of both T cells and monocytes. We have investigated the ability of IL-10 to regulate responses to alloantigens in vitro and in vivo. Addition of IL-10 to mixed lymphocyte cultures profoundly decreased the proliferation and IL-2 production by donor B10.BR cells stimulated with CBA cells expressing minor histocompatibility antigens. Administration of IL-10 for a period of 2 weeks after bone marrow transplantation decreased the expansion of CD4+ and CD8+ donor T cells. In addition, splenocytes from BMT mice treated with IL-10 secreted less IFN-gamma after stimulation with Con A in vitro. The suppression of the mitogen-driven proliferative response of lymphocytes from the IL-10-treated group could also be reversed with significantly less anti-IFN-gamma antibody than for saline-treated controls. However, treatment with IL-10 was not sufficient to alter significantly the clinical course of graft-versus-host disease in CBA recipient mice as assessed by survival, weight loss, and splenomegaly. The results suggest that exogenous IL-10 suppresses the afferent Th1 response in a graft-versus-host reaction but does not significantly diminish the effector stage of graft-versus-host disease.