BACKGROUND: High dose of aprotinin has been known to reduce the requirement for homologous transfusion and the loss of blood during cardiac operation. The aim of this study was to investigate an effective minimal dose of aprotinin. METHODS: With IRB approval 61 patients undergoing cardiac operation were divided into four groups. Group I was control (n=15). Group II(high dose aprotinin, n=16) received an infusion of 24,000 KIU/kg of aprotinin followed by continous infusion of 6,000 KIU/kg per hour until the end of operation. A bolus of 24,000 KIU/kg was added to pump prime solution. Group III(low dose aprotinin, n=16) received a bolus of 24,000 KIU/kg to prime solution. Group IV(minimal dose aprotinin, n=14) received a bolus of 12,000 KIU/kg to prime solution. Coagulation profiles were analysed and blood loss through chest tubes