Mycobacterium paratuberculosis (MPT) is the etiologic agent of paratuberculosis. The disease is prevalent in cattle worldwide, and exacts a heavy financial toll. Effective control requires the development of acellular vaccines offering a better protection than the current available vaccines without side effects and allowing the discrimination between infected and vaccinated animals. We studied the immune response of mice to the MPT superoxide dismutase (SOD) alone or adjuvanted by Ribi. We cloned, overexpressed and purified this antigen in Escherichia coli. Spleen cells from immunized mice, after exposure to recombinant MPT SOD (MPT rSOD), produced significant levels of IFNgamma, TNFalpha and IL-6. IFNgamma and TNFalpha production was increased by the addition of Ribi. In contrast, low levels of NO, IL-4 and IL-10 were secreted by spleen cells culture from immunized mice. The immunoglobulin isotype distribution analysis showed that Ribi adjuvant clearly induced a significantly higher anti-MPT rSOD antibody production of all classes tested and decreased the IgG1/IgG2a ratio thus improving the Th1 response. Delayed-type hypersensitivity responses in mice footpads were observed only in mice immunized with MPT rSOD emulsified in Ribi. Vaccination of MPT rSOD emulsified with Ribi induced both a Th2 and Th1 type of immune response with the later slightly more pronounced. The results presented here on the immunogenicity of MPT SOD suggest that this antigen should be further tested as a candidate antigen for a future acellular vaccine against paratuberculosis.