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1. The dosage of protamine necessary for the postoperative neutralization of heparin, used as anticoagulant during extracorporeal circulation, was studied in 25 patients by means of both the protamine titration test and the plasma thrombin time. 2. The plasma thrombin time was found to be far more sensitive in the detection of small quantities of residual heparin than was the protamine titration test, which failed to demonstrate significant quantities of circulating heparin. 3. Adequate neutralization of circulating heparin was achieved when an initial postoperative protamine dose of 1.5 times the calculated residual heparin was given and followed by additional protamine when indicated by the result of the plasma thrombin time. 4. No toxic effects of protamine were noted on such a dosage schedule.