The 26 patients were randomly selected among patients who underwent valve replacement from February 1992 through September 1993 at National Medical Center. They were divided into two groups, one control group[n=13], the other aprotinin group[n=13]. The aprotinin group recieved 15000 KIU/kg aprotinin in the CPB priming volume and 35000 KIU/kg aprotinin intravenously during operation. We checked preoperative and postoperative hemoglobin, platelet, prothrombin time, activated partial thromboplastin time, creatinine phosphokinase and lactic dehydrogenase. There was no difference in the patient`s above clinical parameters between both groups. The platelet count in both groups decreaced after operation. These findings demonstrated that the effectiveness of aprotinin was not associated with platelet number but probably associated with a protection of platelet function and a prevention of hyperfibrinolysis. The intraoperative and postoperative blood loss and requirement of blood were significantly reduced in aprotinin group.