T helper 1 (T_H1) cells mediate cellular immunity, whereas T_H2 cells potentiate antiparasite and humoral immunity. We used a complementary DNA subtraction method, representational display analysis, to show that the small guanosine triphosphatase Rac2 is expressed selectively in murine T_H1 cells. Rac induces the interferon-γ (IFN-γ) promoter through cooperative activation of the nuclear factor kappa B and p38 mitogen-activated protein kinase pathways. Tetracycline-regulated transgenic mice expressing constitutively active Rac2 in T cells exhibited enhanced IFN-γ production. Dominant-negative Rac inhibited IFN-γ production in murine T cells. Moreover, T cells from Rac2^−/− mice showed decreased IFN-γ production under T_H1 conditions in vitro. Thus, Rac2 activates T_H1-specific signaling and IFN-γ gene expression.