Cholera toxin (CT), a major enterotoxin produced by Vibrio cholerae, is known for its properties as a mucosal adjuvant that promotes Th2 or mixed Th1 + Th2 responses. In this study, we explore the ability of CT to act as a systemic adjuvant to counteract the Th1 response leading to experimental autoimmune uveitis. We report that susceptible B10.RIII mice immunized with a uveitogenic regimen of the retinal Ag interphotoreceptor retinoid-binding protein could be protected from disease by a single systemic injection of as little as 2 micro g of CT at the time of immunization. The protected mice were not immunosuppressed, but rather displayed evidence of immune deviation. Subsequent adaptive responses to interphotoreceptor retinoid-binding protein showed evidence of Th2 enhancement, as indicated by reduced delayed-type hypersensitivity in the context of enhanced Ag-specific lymphocyte proliferation and IL-4 production. Ag-specific production of several other cytokines, including IFN-gamma, was not appreciably altered. The inhibitory effect of CT was dependent on the enzymatic A subunit of CT, because the cell-binding B subunit alone could not block disease development. Mice given CT displayed detectable IL-4 levels in their serum within hours of CT administration. This innate IL-4 production was critical for protection, as infusion of neutralizing Ab against IL-4 to mice, given a uveitogenic immunization and treated with CT, counteracted immune deviation and abrogated protection. Our data indicate that systemic administration of CT inhibits experimental autoimmune uveitis by skewing the response to the uveitogenic autoantigen to a nonpathogenic phenotype.