Since the introduction of the proteinase inhibitor aprotinin in cardiac surgery, a strong increase of the activated clotting time (ACT) during the extracorporeal circulation phase (ECC) was reported in many clinical studies, but with a lack of correlation between ACT and heparin concentration. In searching for a cause of this inconsistency we investigated different surface activators of the ACT in a clinical study. During ECC ACT was measured in parallel, using a Hemochron device and corresponding tubes (nominally 12 mg celite activator) for celite ACT, and a HemoTec device with corresponding double tubes (nominally 0.1 ml kaolin activator) for kaolin ACT. Under the conditions of ECC, the kaolin ACT values (482 +/- 145 sec) were significantly lower than the celite ACT values (985 +/- 267 sec). These results were confirmed in ex-vivo experiments using an activated partial thromboplastin time (aPTT) model. With heparin alone, aPTT activated with celite and kaolin were similar. Including aprotinin in this model, the celite aPTT showed no correlation to the heparin concentration, whereas the kaolin aPTT remained well correlated to the heparin concentration and similar to the values without aprotinin. With aprotinin alone there were no changes of the aPTT times, whereas the celite ACT times were without any correlation. Our results indicate that using kaolin instead of celite the ACT measurements under aprotinin therapy stay in the same ranges as without application of aprotinin: aprotinin has no detectable influence on kaolin-activated ACT. In our opinion, kaolin should be used as the surface activator for ACT measurements under the conditions of ECC, heparinization, and aprotinin therapy.(ABSTRACT TRUNCATED AT 250 WORDS)