Transforming growth factor-β is an essential moderator of malaria-induced inflammation in mice. In this study, we show that the virulence of malaria infections is dependent upon the cellular source of TGF-β and the timing of its production. C57BL/6 mice infected with a nonlethal (Py17X) strain of Plasmodium yoelii produce TGF-β from 5 days postinfection; this correlates with resolution of parasitemia, down-regulation of TNF-α, and full recovery. In contrast, infection with the lethal strain Py17XL induces high levels of circulating TGF-β within 24 h; this is associated with delayed and blunted IFN-γ and TNF-α responses, failure to clear parasites, and 100% mortality. Neutralization of early TGF-β in Py17XL infection leads to a compensatory increase in IL-10 production, while simultaneous neutralization of TGF-β and IL-10R signaling leads to up-regulation of TNF-α and IFN-γ, prolonged survival in all, and ultimate resolution of infection in 40% of Py17XL-infected animals. TGF-β production can be induced in an Ag-specific manner from splenocytes of infected mice, and by cross-linking surface CTLA-4. CD25+ and CD8+ cells are the primary source of TGF-β following Py17X stimulation of splenocytes, whereas Py17XL induces significant production of TGF-β from adherent cells. In mice immunized against Py17XL, the early TGF-β response is inhibited and is accompanied by significant up-regulation of IFN-γ and TNF-α and rapid resolution of challenge infections.