In the mouse, infection with Schistosoma mansoni results in an egg-producing infection and associated disease, whereas vaccination with attenuated larval stages produces a substantial and specific immunity in the absence of egg-induced pathology. Preliminary data showing enhanced interleukin-5 (IL-5) production by T cells from infected mice and interferon gamma (IFN-gamma) synthesis by cells from vaccinated animals (7), suggested differential CD4+ subset stimulation by the different parasite stimuli. To confirm this hypothesis, lymphocytes from vaccinated or infected animals were compared for their ability to produce IFN-gamma and IL-2 (secreted by Th1 cells) as compared with IL-4 and IL-5 (characteristic Th2 cytokines). After stimulation with specific antigen or mitogen, T cells from vaccinated mice or prepatently infected animals responded primarily with Th1 lymphokines, whereas lymphocytes from patently infected mice instead produced Th2 cytokines. The Th2 response in infected animals was shown to be induced by schistosome eggs and directed largely against egg antigens, whereas the Th1 reactivity in vaccinated mice was triggered primarily by larval antigens. Interestingly, Th1 responses in mice carrying egg-producing infections were found to be profoundly downregulated. Moreover, the injection of eggs into vaccinated mice resulted in a reduction of antigen and mitogen-stimulated Th1 function accompanied by a coincident expression of Th2 responses. Together, the data suggest that coincident with the induction of Th2 responses, murine schistosome infection results in an inhibition of potentially protective Th1 function. This previously unrecognized downregulation of Th1 cytokine production may be an important immunological consequence of helminth infection related to host adaptation.