BACKGROUND: High-dose aprotinin appears to enhance the anticoagulant effects of heparin, as documented by increases in the activated clotting times (ACTs) during cardiopulmonary bypass. This increase of the ACT in the presence of aprotinin and heparin is due to the use of celite as surface activator. We compared celite and kaolin as surface activators for the measurement of the ACT in cardiac surgical patients treated with high dose aprotinin. METHODS: This study included 25 patients who were scheduled for coronary bypass graft surgery and reoperation of cardiac valvular surgery. The 2 million units of aprotinin were added to the pump prime of heart-lung machine. The dosage of heparin and protamine was 3 mg/kg respectively. Whole blood was sampled 10 minutes after induction, heparin administration, cardiopulmonary bypass(CPB), 10 minutes before the termination of CPB and 10 minutes after protamine administration. The ACT was measured with Hemochron 801 blood coagulation timer with 12 mg of either celite (C-ACT) or kaolin (K-ACT) used as surface activator. RESULTS: At 10 minutes after induction and heparin administration, celite and kaolin ACTs were l20+/-28, 541+/-247 seconds and 126+/-23, 559+/-267 seconds rcspectively. But 10 minutes after initiation of CPB and before the termination of CPB, celite ACTs were 941+/-238 and 787+/-277 seconds; kaolin ACTs were 605+/-182 and 499+65 seconds, which were consistently less than celite ACTs(p<0.01). At 10 minutes after protamine administration, celite ACT was 118+/-12 seconds and kaolin ACT was 142 56 seconds which was consistently more than celite ACT(p<0.05). CONCLUSIONS: We recommend the K-ACT rather than C-ACT when monitoring of heparin-induced anticoagulation in patients treated with high-dose aprotinin. It is also highly recommended that patients being added with aprotinin should receive the usual doses of heparin and that the ACT should be measured with kaolin as the activator.