Dosage titration of protamine using a heparin dose response curve for the reversal of heparinization after cardiopulmonary bypass and the factors which affect. ACT were investigate. This study included 170 patients undergoing surgery for congenital or acquired heart diseases. Patients were randomly allocated to 6 griyos according to a protamine dosage of either 0.8, 1.0, 1.3, or 1.5 times the residual heparin amounts, or protamine 3mg/kg. The factors affecting ACT which we investigated were the differences between arterial and venous blood, between men and women, between a hematocrit value less of greater than 40%, and between less or more than 2 hours duration of bypass time. The results are as follows: 1) There were no significant differences in postprotamine ACT among the 5 groups. 2) ACT of arterial blood was more prolonged than that of venous blood(139.85±4.77 vs 111.50±2.36 sec). 3) ACT in men was more prolonged than in women(638.81±32.10 vs 559.08±14.33 sec). 4) ACT in which the hematocrit value was less than 40% was more prolonged than that in which it was above 40%. 5) Although there was no difference between less and more than 2 hours duration of bypass time in ACT, additional protamine was needed in latter group.