IL-10 is an immunoregulatory cytokine that can modulate immune processes, inhibiting the expression of inflammatory T(h)1 type responses as well as affecting antigen-presenting cell function. In addition, IL-10 has been shown to be active at mucosal surfaces. In the present study, we examined the role of IL-10 on orally and nasally induced tolerance. Treatment of (PL/J x SJL)F(1) mice with low-dose oral myelin basic protein (MBP) (0.5 mg) and simultaneous oral IL-10 given 3 times reduced the severity and incidence of experimental autoimmune encephalomyelitis (EAE), whereas administration of oral IL-10 alone or MBP alone given in these doses had no effect. Lymphocytes from mice treated orally with MBP and IL-10 proliferated less, and produced decreased amounts of IFN-gamma and IL-2 and increased amounts of IL-10 and transforming growth factor-beta upon in vitro stimulation with MBP. Nasal administration of antigen and IL-10 reduced proliferative responses and IFN-gamma production, increased IL-10 production, and enhanced protection from EAE. In addition, oral IL-10 combined with oral myelin oligodendrocyte glycoprotein (MOG) 35-55 reduced relapses in MOG-induced EAE in the NOD mouse, as well as enhanced the protective effect of oral insulin in the NOD model of diabetes. These results demonstrate that IL-10 is biologically active at mucosal surfaces and can act synergistically to enhance the tolerogenic effects of mucosally administered antigen.