The evidence is reviewed and a model presented for two distinct receptors being involved in platelet activation induced by alpha-thrombin: a high affinity thrombin receptor constituting approximately 50 copies of a supercomplexed form of GPIb coupled to phospholipase A2 and a moderate affinity receptor constituting approximately 2000 copies of the proteolytically activated, G protein-coupled seven transmembrane domain receptor coupled to phospholipase C. Reasons for the failure of certain studies to detect this role for GPIb are discussed.