In addition to classic Smad signaling pathways, the pleiotropic immunoregulatory cytokine TGF-β1 can activate MAP kinases, but a role for TGF-β1-MAP kinase pathways in T cells has not been defined heretofore. We have shown previously that TGF-β1 inhibits Th1 development by inhibiting IFN-γ's induction of T-bet and other Th1 differentiation genes, and that TGF-β1 inhibits receptor-proximal IFN-γ-Jak-Stat signaling responses. We now show that these effects of TGF-β1 are independent of the canonical TGF-β1 signaling module Smad3, but involve a specific MAP kinase pathway. In primary T cells, TGF-β1 activated the MEK/ERK and p38 MAP kinase pathways, but not the JNK pathway. Inhibition of the MEK/ERK pathway completely eliminated the inhibitory effects of TGF-β1 on IFN-γ responses in T cells, whereas inhibition of the p38 pathway had no effect. Thus, TGF-β1's inhibition of IFN-γ signaling in T cells is mediated through a highly specific Smad3 independent, MEK/ERK-dependent signaling pathway.