In the present study the thromboresistance of heparin-coated diffusion membrane oxygenators (Jostra, M 30) combined with heparin-coated venous reservoirs, tubing sets and arterial filters was investigated in six bovine experiments (70 +/- 5 kg). The perfusion with reduced systemic heparin dose (100 IE/kg) body weight) was performed with activated clotting time over 180 seconds. The perfusion began with a blood flow of 31/min and was maintained during six hours. Clotting studies including blood platelet count, activated clotting time, fibrinogen (factor I), antithrombin III and fibrinopeptid A were performed before the operation and ten minutes, two hours and five and six hours after beginning of bypass. The venous and arterial saturation remained stable during the whole investigation. After ten minutes activated clotting time dropped from 619 +/- 114s to 203 +/- 15s after six hours (p < 0.05). The antithrombin III level changed significantly from 109 +/- 11% to 95 +/- 16%. Factor 1 and fibrinopeptid A changes were not significant: from 1.6 +/- 0.3 g/1 to 1.5 +/- 0.3 g/1, and 3.0 +/- 1.4 ng/mL to 3.5 +/- 1.2 ng/mL, accordingly. There were no mechanical defects and especially no plasma leakage. Slight sediments were found only in areas of stagnant blood flow. The investigated bypass circuit with reduced systemic heparinization seems therefore particularly appropriate for long-term perfusions.