Program death-1 (PD-1), an inhibitory receptor upregulated on T cells upon TCR stimulation, has been shown to attenuate a number of immune responses in vivo, including acute allograft rejection. We tested whether constitutive expression of PD-1 would further inhibit allograft rejection. To this end, we generated transgenic mice expressing T-cell-restricted PD-1 under the control of the Lck proximal promoter and CD2 locus control. PD-1 transgenic (PD-1-Tg) mice did not develop gross abnormalities of thymic development and displayed normal numbers of thymocyte subsets and peripheral T cells. In vitro, PD-1-Tg T cells had reduced proliferative and cytokine secretion capacity upon TCR stimulation and cross-linking of PD-1 resulted in diminished phosphorylation of protein kinase C-theta and Akt, as well as increased activation of the phosphate and tensin homolog. However, only T-cell responses to minor but not major mismatches were reduced in vitro. Similarly, PD-1-Tg mice exhibited prolonged survival of cardiac allografts only in mice transplanted with heart allografts expressing multiple minor mismatches and treated with suboptimal doses of cyclosporine A. We conclude that genetic engineering of T cells to express PD-1 constitutively has only a mild impact on allograft survival.