African trypanosomiasis encompasses diseases caused by pathogenic trypanosomes, infecting both humans and animals. In the present article, we dissected the possible role of members of the interleukin (IL)-12 family during infection with Trypanosoma brucei brucei and Trypanosoma evansi in mice. IL-12p35(-/-), IL-12p40(-/-), and IL-12p35(-/-)/p40(-/-) mice were susceptible to both pathogens, as was demonstrated by the increased mortality among these mice, compared with wild-type C57BL/6 mice. The different IL-12p70(-/-) mouse strains showed similar mortality kinetics, suggesting that IL-12p70--but not the IL-12p80 homodimer or IL-23--plays a crucial role in survival. Although there were similar plasma levels of immunoglobulin (Ig) M and IgG2a in IL-12-deficient mice and wild-type mice, interferon (IFN)- gamma production, especially during early infection, was severely impaired in all IL-12p70(-/-) mouse strains, demonstrating an IL-12p70-dependent mechanism for IFN- gamma production. Because IFN- gamma receptor-deficient mice (IFN- gamma R(-/-)) were also highly susceptible to both Trypanosoma species, IL-12p70-dependent IFN- gamma production seems to be the important mechanism involved in resistance against both pathogens.