The glucocorticoid-induced TNF receptor (GITR), which is a member of the TNF receptor family, is expressed preferentially at high levels on CD25+ CD4+ regulatory T cells and plays a key role in the peripheral tolerance that is mediated by these cells. GITR is also expressed on conventional CD4+ and CD8+ T cells, and its expression is enhanced rapidly after activation. In this report we show that the GITR provides a potent costimulatory signal to both CD25+ and CD25− CD4+ T cells. GITR-mediated stimulation induced by anti-GITR mAb DTA-1 or GITR ligand transfectants efficiently augmented the proliferation of both CD25− CD4+ and CD25+ CD4+ T cells under the limited dose of anti-CD3 stimulation. The augmentation of T cell activation was further confirmed by the enhanced cell cycle progression; early induction of the activation Ags, CD69 and CD25; cytokine production, such as IL-2, IFN-γ, IL-4, and IL-10; anti-CD3-induced redirected cytotoxicity; and intracellular signaling, assessed by translocation of NF-κB components. GITR costimulation showed a potent ability to produce high amounts of IL-10, which resulted in counter-regulation of the enhanced proliferative responses. Our results highlight evidence that GITR acts as a potent and unique costimulator for an early CD4+ T cell activation.