To observe the protective effect of heparin-coated circuits (HCC) on the platelet function during cardiopulmonary bypass (CPB), 23 patients with heart valve replacement were studied. The system heparin dose was 3 mg/kg in the control group (n = 15) and heparin-coated circuits in the HCC group (n = 8). Platelet count, alpha-granule membrane protein-140 (GMP-140) concentrations were determined before CPB, at 60 min of CPB, 30 and 60 min after protamine administration, first 12 h after CPB, respectively. At end of CPB the arterial filters in the circuits were observed by electron microscopy. The amount of first 12-h postoperative blood loss was measured. There was significant reduction in platelet loss during and after CPB in the HCC group in contrast to the control group during CPB (P<0.05). During the first 12 h, postoperative blood loss was reduced in the HCC group as compared with that in the control group (218+/-61 ml, vs. 332+/-118 ml, P<0.05). Electron microscopy showed that in the HCC group the filter meshes and their fringes were clear and fragments of floccules were occasionally seen, without adherent cells or only few adherent cells on their surfaces, whereas several cellular and fibrous components were found to adhere to the surfaces of the filter meshes in the control group. This study indicates that heparin-coated circuits might reduce the platelet loss and activation during CPB and improve hemocompatibility of cardiopulmonary bypass equipment.