The role of virulent forms of Trypanosoma cruzi in modulating mitogen-induced lymphocyte responses was investigated in this work. Bloodstream forms of T. cruzi inhibited normal mouse spleen cell responses to Con A and LPS in a dose-dependent manner. Reduced responses were observed over relatively large ranges of concentration of Con A (50-fold) and LPS (160-fold). The inhibitory action of the parasites could not be overcome by increasing the mitogen dose beyond optimal levels. Furthermore, absorption of mitogen solutions with four times as many parasites as used in the proliferation assays revealed that sufficient mitogen activity remained to produce optimal lymphocyte responses. Therefore, reduced lymphocyte responsiveness was not due to absorption of mitogen by the parasite. Inhibited responses were also seen when a sonicated T. cruzi preparation was used, indicating that parasite viability was not required to produce suppression. Inhibition of Con A- or LPS-induced responses by the parasites occurred only when the trypanosomes were incorporated into the system during the first 24 hr of culture. These results show that virulent forms of T. cruzi can induce suppression of T and B cell responses in vitro, and suggest that the parasite affects lymphocyte commitment to blastogenesis during the early stages of lymphocyte activation.