Objective The aim of this study was to determine the effects of ulinastatin, a broad spectrum proteinase inhibitor, on fibrinolytic system and platelet function during open heart surgery performed with cardiopulmonary bypass (CPB) .Methods Twenty ASA Ⅰ-Ⅱpatients of both sexes undergoing cardiac surgery under CPB were randomly divided into two groups of ten patients: control group(C) and ulinastatin group (U). In group U patients received ulinastatin 12 000 U·2kg-1 . Half of the dose was given iv 10 min before CPB and the other half was added to the priming solution. In group C patients received normal saline instead of ulinastatin. Blood samples were taken before CPB (T1 ) , 30 min after CPB was started (T2), at the end of CPB (T3), 2 h and 4 h after CPB(T4 , T5) for determination of plasma levels of D-Dimer, α-granule membrane protein-140 (GMP-140), thromboxane B2 (TXB2) and 6-Keto-prostaglandin F1α (6-Keto-PGF1α) .Results The demographic data, aortic cross-clamping time, CPB time and duration of operation were comparable between the two groups. The plasma levels of D-Dimer, GMP-140, TXB2 and TXB2/6-Keto-PGFlα were significantly increased at T2 , T3 and T4 as compared with the baseline (T1 ) in both groups, but the increase was significantly larger in group C than in group U(P<0.05 or 0.01). There was significantly less postoperative blood loss in group U ( P < 0.05) Conclusion CPB results in increased fibrinolysis and decreased platelet function. Ulinastatin is effective in stabilizing fibrinolytic system and preserving platelet function during CPB, leading to less postoperative blood loss.