Heparin levels were followed by an amidolytic method during open heart surgery in 10 adult patients and correlated to the activated clotting time (ACT) (Haemochron). There was a good correlation between the 2 parameters when the ACT was below 600 s. Based on the present and previous studies of the ACT the authors conclude that the ACT is a useful tool in control of heparinization during open heart surgery.