We studied 37 patients in order to evaluate the heparin-neutralizing efficacy of the protamine dose calculated by either the fixed dose method or the dose response curve method on the activated coagulation time(ACT) during open heart surgery. According to the method calculating the protamine dose, the patients were randomly allocated to receive the protamine dose calculated by either the fixed dose method(N=17) or the dose response curve method(N= 20). The ACT was measured at immediately after sternotomy(baseline ACT) and 5 minutes after protamine administration(post-protamine ACT). There were no significant differences in baseline ACT or post-protamine ACT values between the two groups. The baseline ACT values in each group were not significantly different from those of post-protamine ACT. The patient numbers of post-protamine ACT values over 130 seconds and the differences from baseline ACT to post-protamine ACT were not significantly different between both groups. The results suggest the a dose of protamine sulfate calculated by a fixed dose method instead of a dose response curve method is available for neutralizing a heparin effect during open heart surgery.