In this study, we describe the generation and characterization of mice in which GITR gene (TNFRSF18 [tumor necrosis factor receptor superfamily 18]), a member of the TNFRSF expressed mainly on T lymphocytes, has been ablated (GITR(-/-) mice). Results indicate that GITR inactivation does not impair the normal development of the lymphoid organs but modulates T-cell activation. In fact, when GITR(-/-) T lymphocytes are activated by treatment with an anti-CD3 monoclonal antibody they proliferate more than wild-type cells. Moreover, activated GITR(-/-) T lymphocytes express higher levels of interleukin-2 receptor, produce larger amounts of interleukin-2, and are more sensitive to activation-induced cell death than controls. These results suggest that GITR is involved in the regulation of T-cell receptor/CD3-driven T-cell activation and programmed cell death.