Background— In vivo testing of novel antiplatelet agents requires informative biomarkers. By genetically modifying mouse von Willebrand factor (VWF R1326H ), we have developed a small animal model that supports human but not mouse platelet-mediated thrombosis. Here, we evaluate the use of this biological platform as a pharmacodynamic biomarker for antithrombotic therapies. Methods and Results— The antithrombotic effects of several αIIbβ3 inhibitors were determined in VWF R1326H mutant mice infused with human platelets. Administration of abciximab, eptifibatide, or tirofiban at doses recommended for percutaneous coronary intervention (per 1 kg of body weight) significantly reduced human platelet-mediated thrombus formation in laser-injured arterioles by >75% ( P <0.001). In contrast, clot size in wild-type control animals remained essentially unchanged ( P >0.05), results consistent with observed species differences in IC 50 values obtained by aggregometry. To further demonstrate that our biological platform is unique among standard mouse models, we evaluated the thrombogenic potential of platelets from healthy volunteers before and after clopidogrel therapy. Consistent with the antithrombotic effect of this agent, platelets postdrug administration formed smaller thrombi than cells before therapy and were less responsive to ADP-induced aggregation ( P <0.001). Conclusions— The ability of αIIbβ3 and P2Y 12 inhibitors to limit human platelet clot formation at doses recommended by the American College of Cardiology/American Heart Association suggests that VWF R1326H mutant mice can serve as both a pharmacodynamic and a functional response biomarker, attributes essential for not only expediting drug development but also designing clinical studies.