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A glycoprotein of human platelets is released into the incubation medium upon treatment of the intact cells with 1 unit per ml of thrombin for 2 min. The cellular location and properties of this protein differ from those of other thrombin-labile platelet proteins such as fibrin-stabilizing factor, fibrinogen, and the contractile protein thrombosthenin. It occurs only in the particulate fraction and is released only from intact cells. Thrombin concentrations of 100 units per ml do not release this thrombin-sensitive protein from sonicated platelets. As isolated from the incubation supernatant of thrombin-treated platelets the thrombin-sensitive protein is excluded from Sephadex G-200; it has an apparent molecular weight of 190,000 in sodium dodecyl sulfate polyacrylamide gels, from which it has been purified and partially characterized. No smaller subunits have been found. It has limited solubility in the absence of detergent, and is highly sensitive to proteolytic degradation in whole platelets and after release from the cells.