Mice infected with Plasmodium yoelii (265 BY, a nonlethal strain) after recovering from parasitemia become resistant to reinfection. In the present study, we have attempted to define the role of T cell subsets in primary vs secondary P. yoelii infection. We have evaluated the in vivo effects of selective depletion of each subset of T cells on the course of infection and also investigated the in vitro expansion of each subset in response to homologous antigen. Depletion of CD4- or CD8-bearing T cells did not result in reappearance of parasitemia in animals cured from primary infection. However, 25% of reinfected animals treated with anti-CD4 mAb, but not with anti-CD8 mAb, displayed a low level (2 to 3%) of parasitemia late in the secondary infection. The splenocyte response to P. yoelii antigen or to T-cell mitogens was impaired during patient infection, even in the 25% of CD4-depleted animals with low parasitemia. A markedly high lymphocyte reactivity to antigen was observed in mice recovered from primary infection, and this was enhanced in animals exposed to a challenge infection. In the case of animals cured from primary infection, a marked decrease in antigen-induced in vitro lymphocyte proliferation occurred in CD8-depleted but not in CD4-depleted animals when total splenic cell populations were assayed. Although less dramatic, a significant diminution in antigen reactivity was observed also with T-rich populations in CD8-depleted animals. In contrast, there was no such decrease in CD4-depleted mice. Treatment of resistant animals which were challenged with a second infection, with anti-CD4 or anti-CD8 mAbs, resulted in a significant decrease in the proliferative response to antigen by both T-rich and total cell populations. In the secondary infection, the T-rich cell populations from CD8-depleted mice responded better to P. yoelii antigen than the total cell populations, indicating an inhibitory role on the expansion of CD4+ T cells of B cells or their product(s) which were removed during T cell enrichment. The results of our study suggest that CD8-bearing T cells were more reactive in the primary infection. In the secondary infection, although both CD8+ and CD4+ T cells were antigen reactive, the latter T cell subset appeared to play a superior role in controlling the parasitemia.