The mechanisms that regulate CD4+ T cells responses in vivo are still poorly understood. We show here that initial Ag stimulation induces in CD4+ T cells a program of proliferation that can develop, for at least seven cycles of division, in the absence of subsequent Ag or cytokine requirement. Thereafter, proliferation stops but can be reinitiated by novel Ag stimulation. This initial Ag stimulation does not however suffice to induce the differentiation of naive CD4+ T cells into effector Th1 cells which requires multiple contacts with Ag-loaded APC. Thus, recurrent exposure to both Ag and polarizing cytokines appears to be essential for the differentiation of IFN-γ-producing cells. Ag and cytokine availability therefore greatly limits the differentiation, but not the initial proliferation, of CD4+ T cells into IFN-γ-producing cells.