Reciprocal differentiation of immunosuppressive CD4+CD25+FoxP3+ T regulatory cells (Tregs) andproinflammatory IL-17-producing cells (Th17) from naïve CD4 cells is contingent upon the cytokineenvironment. Using MACS-purified CD4 cells, we found that rapamycin and cyclosporine A (CsA)potently inhibited the TGFβ and IL-6-induced generation of IL-17-producing cells. Intriguingly,rapamycin promoted, while CsA markedly inhibited, TGFβ-mediated generation of Tregs. Theaforementioned effects of rapamycin and CsA were also observed for Flow-sorted CD4+CD25− Tcells, indicating that the effect of these two immunosuppressive agents was based on their action onde novo generation of Tregs and Th17 cells from naïve CD4 cells. Our observation suggests a distinctmode of immunosuppressive action and tolerance induction by rapamycin and CsA. The capacity ofrapamycin to generate immunosuppressive Tregs and to suppress differentiation of pathogenic Th17cells furthers our understanding of the basis for the therapeutic immunosuppressive effects ofrapamycin in patients with autoimmune diseases and allo-transplantation reactions.