In a prospective study of 13 patients undergoing open-heart surgery with extracorporeal circulation, marked qualitative platelet function defects were observed in addition to the usually occurring drop of the thrombocyte count. At the end of bypass, the following test results were significantly abnormal: concentration of fibrinogen and of circulating fibrin degradation products, platelet count, platelet adhesiveness to glass beads, and platelet aggregation induced by low and high doses of ADP. One to 2 hours after neutralization of heparin with protamine sulfate all abnormal test results improved, but the template bleeding time was markedly prolonged in 10 patients. There was no correlation between length of bypass and platelet fall and between concentration of circulating fibrin degradation products and extent of platelet dysfunction. An apparent correlation was found between the length of the postoperative bleeding time and the number of units of blood transfued during surgery. The results of this study suggest that dilution of the patient's own platelets by nonviable platelets contained in 3-day-old transfused ACD blood and the production of a refractory state of the patient's circulating platelets to ADP induced aggregation played a significant role in the development of platelet function abnormalities during extracorporeal circulation.