Platelet transfusion and aprotinin administration improve platelet function and clinical hemostasis after extracorporeal circulation. To compare two methods of improving postoperative hemostasis, we preoperatively randomized 40 patients undergoing various open heart procedures into two groups. Group A included 20 patients who, immediately after bypass, received single-donor plateletpheresis concentrates collected from ABO-compatible donors (Baxter Autopheresis-C System). They were compared with 20 patients who received high-dose aprotinin (6 × 106 KIU) before and during cardiopulmonary bypass (group B). Group A patients showed significantly higher platelet count after single-donor plateletpheresis concentrate transfusion (157 ± 36 × 109/L compared with 118 ± 42 × 109/L (p < 0.05). However, platelet aggregation on extracellular matrix was better in group B (3.4 ± 0.7 versus 2.8 ± 0.9; p < 0.05). Total 24-hour blood loss and exposure to homologous blood products were significantly less in group B (396 ± 125 mL and 1.1 ± 1.6 units compared with 617 ± 233 mL and 5.4 ± 3.4 units; p < 0.01). Despite higher platelet count in patients after single-donor plateletpheresis concentrates transfusion, hemostasis in patients receiving aprotinin is better due to improved platelet function.