A new lineage of effector CD4+ T cells characterized by production of interleukin (IL)-17, the T-helper-17 (TH17) lineage, was recently described based on developmental and functional features distinct from those of classical TH1 and TH2 lineages. Like TH1 and TH2, TH17 cells almost certainly evolved to provide adaptive immunity tailored to specific classes of pathogens, such as extracellular bacteria. Aberrant TH17 responses have been implicated in a growing list of autoimmune disorders. TH17 development has been linked to IL-23, an IL-12 cytokine family member that shares with IL-12 a common subunit, IL-12p40 (ref. 8). The IL-23 and IL-12 receptors also share a subunit, IL-12Rβ1, that pairs with unique, inducible components, IL-23R and IL-12Rβ2, to confer receptor responsiveness. Here we identify transforming growth factor-β (TGF-β) as a cytokine critical for commitment to TH17 development. TGF-β acts to upregulate IL-23R expression, thereby conferring responsiveness to IL-23. Although dispensable for the development of IL-17-producing T cells in vitro and in vivo, IL-23 is required for host protection against a bacterial pathogen, Citrobacter rodentium. The action of TGF-β on naive T cells is antagonized by interferon-γ and IL-4, thus providing a mechanism for divergence of the TH1, TH2 and TH17 lineages.