THROMBIN, a coagulation protease generated at sites of vascular injury, activates platelets, endothelial cells, leukocytes and mesenchymal cells(1,2). A G-protein-coupled receptor that is proteolytically activated by thrombin(3) is a target for drug development aimed at blocking thrombosis, inflammation and proliferation. Here we show that although disruption of the thrombin-receptor (tr) gene in mice causes about half the tr(-/-) embryos to die at embryonic day 9-10, half survive to become grossly normal adult mice with no bleeding diathesis. Strikingly, tr(-/-) platelets respond strongly to thrombin, whereas tr(-/-) fibroblasts lose their ability to respond to thrombin. We conclude that the thrombin receptor plays an unexpected role in embryonic development, suggesting a possible new function for the ’coagulation’ proteases themselves. Moreover, a second platelet thrombin receptor exists, and different thrombin receptors have tissue-specific roles. This may allow development of therapeutics that will selectively block thrombin’s different cellular actions.