—An antibody was raised in rabbits against SFFLRNPSEDTFEQF peptide, which is an NH₂-terminal peptide of the thrombin-cleaved rat thrombin receptor. In vitro, the antibody inhibited rat smooth muscle cell proliferation but had no effect on rat platelet aggregation or clotting time. These data indicate that the antibody is a specific blocker of the thrombin receptor–signaling pathway in rat smooth muscle cells but does not work as a blocker in rat platelets, suggesting the existence of a second thrombin receptor in the platelets. Using an in vivo balloon catheter–induced injury model in rats, we examined the effect of the anti-rat thrombin receptor IgG on intimal smooth muscle cell accumulation 2 weeks after angioplasty. Analysis of the ratio of intimal to medial cross-sectional areas showed that injection of immune IgG resulted in 43.7% and 53.1% reduction (P<0.01) of neointimal smooth muscle cell accumulation compared with saline and nonimmune IgG treatment, respectively. Moreover, the injection of immune IgG caused a significant decrease of thrombin receptor mRNA expression and also 40.5% and 43.0% decreases (P<0.01) of the proliferating cell nuclear antigen (PCNA) index in the intima compared with the PCNA index after saline and nonimmune IgG treatment, respectively. The suppression of the PCNA index was also observed in the immune IgG–treated group at an early stage after angioplasty. These results suggest that thrombin receptor activation is involved in the proliferation and accumulation of neointimal smooth muscle cells induced by balloon injury.