Mycobacterium paratuberculosis (MPT) is the etiological agent of paratuberculosis. The disease is prevalent throughout the world, and exacts a heavy financial toll. At present, the only means of controlling this disease are culling or vaccination. The existing vaccines are not very efficient and produce a long-lasting local reaction at the point of injection and induce anti-bodies/delayed-type hypersensitivity (DTH) reaction that cannot be differentiated from those of naturally infected animals. New potent acellular vaccines that allow discrimination between infected and vaccinated animals are necessary to improve the control of this disease. We have isolated, overexpressed and purified the 85B antigen of MPT, and characterized the immune response induced by this antigen in mice. Our results showed that the recombinant MPT 85B (rMPT 85B) antigen induced a high production of interferon (IFN)gamma, interleukin (IL)-6, IL-10 and nitric oxide (NO). Spleen cells from mice immunized with rMPT 85B in Ribi adjuvant produced a higher level of IL-10 and NO than spleen cells of mice immunized with rMPT 85B only. In contrast, the addition of Ribi to the immunization protocol resulted in a lower amount of IFNgamma released by spleen cells. The levels of spleen cells proliferation in mice vaccinated with the rMPT 85B protein alone or with rMPT 85B with Ribi adjuvant were, respectively, four times or five times greater than in the control mice. The Ribi adjuvant induced significantly higher anti-85B antibody production of all classes tested and increased the IgG1/IgG2a ratio. DTH responses in mice footpads were observed only in mice immunized with rMPT 85B emulsified in Ribi. rMPT 85B induced both a Th1 and Th2 type of immune response with the later slightly more pronounced when the vaccination protocol comprised Ribi as an adjuvant. The rMPT 85B antigen elicited a strong immune response and can be considered as a potential candidate for a future acellular vaccine.