Th cytokines IFN-γ and IL-17 are linked to the development of autoimmune disease. In models of rheumatoid arthritis, that is, proteoglycan (PG)-induced arthritis, IFN-γ is required, whereas in collagen-induced arthritis, IL-17 is necessary for development of arthritis. In this study we show that the route of immunization determines the requirement for either IFN-γ or IL-17 in arthritis. Intraperitoneal immunization with PG induces a CD4(+) T cell IFN-γ response with little IL-17 in the spleen and peripheral lymph nodes. However, s.c. immunization induces both an IFN-γ and an IL-17 CD4(+) T cell response in spleen and lymph nodes. The failure to induce a CD4(+) T cell IL-17 response after i.p. immunization is associated with T cell priming, as naive T cells activated in vitro were fully capable of producing IL-17. Moreover, PG-induced arthritis is converted from an IFN-γ to an IL-17-mediated disease by altering the route of immunization from i.p. to s.c. The histological appearance of joint inflammation (cellular inflammation and bone erosion) is similar in the i.p. versus s.c. immunized mice despite the presence of CD4(+) T cells producing IL-17 in joint tissues only after s.c. immunization. These data indicate a critical role for the site of initial T cell priming and the Th cytokines required for susceptibility to arthritis. Our findings suggest that T cell activation at different anatomical sites in rheumatoid arthritis patients may skew the T cells toward production of either IFN-γ or IL-17.