IL-2 and TGF-β both have important roles in the induction and maintenance of immunologic tolerance, but whether these cytokines act separately or together to achieve this effect is poorly understood. Although others have reported that IL-2 can directly enhance forkhead box protein P3 (Foxp3) transcription factor expression by natural CD4+ CD25+ regulatory T cells, in this study, we report that the role of IL-2 on the generation of peripheral regulatory CD4+ cells is indirect. Ab neutralization studies and experiments with IL-2-deficient mice have revealed that IL-2 is required for TGF-β to induce naive CD4+ CD25− cells to become CD25+ and express Foxp3, and develop the characteristic properties of CD4+ CD25+ regulatory cells. This effect of IL-2 on the generation and expansion of these adaptive Foxp3+ regulatory cells is nonredundant, but IL-4, IL-7, and IL-15, other common γ-chain cytokines, could sustain Foxp3 expression. Because subjects with autoimmune diseases often have defects in the production of IL-2 and/or TGF-β, the generation of autologous T regulatory cells ex vivo with these cytokines for transfer in vivo may have considerable therapeutic potential.