Platelet membrane glycoprotein V (GPV) was hydrolyzed during thrombin-induced platelet aggregation releasing a fragment GPVfl into the supernatant. Hydrolysis of GPV required catalytically active thrombin and was diminished by chemical modification of the fibrinogen binding site of thrombin. Half-maximal liberation of GPVfl occurred at a 10-fold higher concentration of thrombin than was required for half-maximal release. Time course studies at several thrombin concentrations showed disparate release of GPVfl and thrombospondin. These results emphasize the complexity of the initial events in thrombin-induced platelet activation.