The role of variant surface glycoprotein (VSG)-specific Th cell responses in determining resistance to the African trypanosomes was examined by comparing Th cell responses in relatively resistant and susceptible mice as well as in cytokine gene knockout mice infected with Trypanosoma brucei rhodesiense. Resistant B10. BR and C57BL/6 mice expressed Th1 cell cytokine responses to VSG stimulation during infection, while susceptible C3H mice produced weak or no Th1 cell cytokine responses. Neither resistant B10. BR and C57BL/6 mice nor susceptible C3H mice made detectable Th2 cell cytokine responses to parasite Ag. To more closely examine the potential role of IFN-γ and other cytokines in host resistance, we determined the resistance phenotypes and Th cell responses of IFN-γ and IL-4 knockout mice. Infected C57BL/6-IFN-γ knockout mice were as susceptible as C57BL/6-scid mice and made an IL-2, but not an IL-4, cytokine response to VSG, while C57BL/6-IL-4 knockout mice were as resistant as the wild-type strain and exhibited both IL-2 and IFN-γ cytokine responses. Passive transfer of spleen cells from wild-type mice to IFN-γ knockout mice resulted in enhanced survival. Both wild-type and IFN-γ knockout mice controlled parasitemia with VSG-specific Ab responses, although parasitemias were higher in the IFN-γ knockout mice. Overall, this study demonstrates for the first time that relative resistance to African trypanosomes is associated with a strong Th1 cell response to parasite Ags, that IFN-γ, but not IL-4, is linked to host resistance, and that susceptible animals do not make compensatory Th2 cell responses in the absence of Th1 cell cytokine responses.