Objective To compare the effects of acidium para-aminomethylben zoicum (APAMZ) and aprotinin on the protection of platelets during cardiopulmonary bypass (CPB). Methods Platelet counts and amount of blood loss and transfusion were recorded after CPB, GMP-140, TXB_2 and 6-K- PGF_(1α)assessed before and after CPB in the APAMZ group and aprotinin group. Results No significant difference existed between the two groups in platelet counts, amount of blood loss and transfusion needed, and plasma GMP-140, TXB_2 and 6-K- PGF_(1α)(t=0.65-1.70,P0.05). Conclusion APAMZ and aprotinin have similar protecting effects on platelets during CPB.