The serine protease, thrombin, plays a crucial role in both coagulation and platelet activation. Anhydrothrombin (AhT) is a catalytically inactive derivative of thrombin in which dehydroalanine replaces the active-site serine. AhT retains affinity for natural substrates of thrombin and may be a competitive inhibitor of thrombin-mediated coagulation and platelet reactions. In the present study, thrombelastography showed that AhT not only delayed the onset and the progress of the coagulation process but impaired clot strength, indicating that AhT may have both anticoagulant and antiplatelet activity. In addition, AhT prolonged the activated partial thromboplastin time dose-dependently, but had little effect on the prothrombin time, suggesting that its principal activity was mediated in the intrinsic coagulation pathway. AhT inhibited thrombin-induced aggregation of platelet-rich plasma. Complete inhibition of aggregation was evident at a concentration of 1.85 microM AhT. Furthermore, 3.7 microM of AhT almost completely abolished shear-induced platelet aggregation in PRP. Interpretation of this in vitro study requires confirmation in vivo, but the findings suggest that thrombin plays a critical role in shear related platelet mechanisms. AhT may be a useful tool for investigating platelet-based coagulation reactions and may provide the basis for a novel class of antithrombotic agents.