Although it is reported that postoperative bleeding is reduced by reinfusing autologous platelet-rich plasma (PRP) after cardiopulmonary bypass (CPB), the effect of PRP on hemostasis is not reported in detail. We prepared PRP and fresh whole blood (WB) from the blood of seven patients each prior to their undergoing CPB, and reinfused them autologously to the patients intravenously after the CPB was terminated. In this article, the effect on hemostasis of autologous PRP and WB was described. Platelet aggregation rates and blood coagulation factors were examined before, during and after bypass. Platelet counts, ADP-induced platelet aggregation and the activities of coagulation factors II, V and VII-X were significantly greater in prepared PRP than in WB (p less than 0.01 or p less than 0.05). A mean volume of 724 +/- 109 ml of PRP or 401 +/- 63 ml of WB was reinfused within about 30 minutes after heparin was neutralized by protamine sulfate. The platelet counts increased from 4.3 +/- 1.4 x 10(4)/mm3 to 14.1 +/- 1.6 x 10(4)/mm3 after PRP reinfusion and the platelet aggregation rates increased significantly (p less than 0.01) after PRP reinfusion compared to WB transfusion. The activities of coagulation factors VII-X also increased significantly (p less than 0.05) after reinfusion of PRP when compared to transfusion of WB. The activated partial thromboplastin time decreased to 1.2 times the baseline in the PRP group but remained 1.5 times the baseline in the WB group (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)