T cell responses to the variant surface glycoprotein (VSG) previously have not been detected in animals infected with the African trypanosomes despite the fact that such animals make strong T-dependent B cell responses to VSG molecules displayed by the parasites. In the present study, we have examined B 10.BR mice for VSG-specific Th cell responses at different times after infection with Trypanosoma brucei rhodesiense clone LouTat 1. T cell populations derived from different tissues were tested for their ability to proliferate and secrete cytokines when stimulated with purified LouTat 1 VSG. Furthermore, VSG-specific T cell lines and clones were derived from immunized mice and examined for their phenotypic and functional profiles in comparison with T cell responses of infected mice. The results of this study show that VSG-specific T cells were not consistently detected in the peripheral lymphoid tissues such as spleen or lymph nodes of infected animals. In contrast, VSG Ag-specific T cells were detectable principally in the peritoneal T cell populations of infected mice. Peritoneal T cells did not proliferate in response to VSG, yet produced substantial cytokine responses when stimulated; the cytokines produced were IFN-gamma and IL-2, without detectable IL-4. The cellular phenotype of VSG-responsive T cells was that of classical Th cells in that all cells were CD4-positive and expressed the CD3 alpha/beta TCR membrane complex. Thus, the VSG appears to preferentially stimulate a Th1 cell subset response during infection. Intrinsic molecular characteristics of the VSG molecule did not induce mice to make this response, however, since VSG-specific T cell lines derived from VSG-immunized mice displayed cytokine profiles characteristic of both Th1 and Th2 cells. Isolation of Th1 clones from selected lines demonstrated that these cells displayed the same membrane-phenotypic characteristics and cytokine profiles as the T cells from infected mice. Furthermore, all Th clones were VSG type-specific, APC-dependent, and I-Ak-restricted in their responses. In summary, these experiments provide the first direct evidence for VSG-specific responses at the T cell level. T cell responses to the VSG molecule during infection appear to be anatomically compartmentalized and exhibit evidence of clonal maturation (cytokine production) but not clonal expansion (proliferation) after antigenic stimulation. The cellular phenotype and cytokine profiles predict that infection predisposes the animals to mount Th1 cell subset responses to VSG. The results of this study, including the T clones generated, provide an experimental basis for examining the regulation of VSG-specific immune responses during infection.