Dose-response effects of heparin and protamine in 34 adult patients undergoing cardiac operations were monitored by an in vitro analysis utilizing hexadimetharine bromide (Polybrene) neutralization. Heparin administered prior to cannulation for cardiopulmonary bypass in a dose of 3.0 mg (300 units) per kilogram of body weight, and 1.5 mg (150 units) per kilogram for each subsequent hour of bypass, routinely produced circulating heparin concentrations greater than 1.0 units per milliliter of plasma. A protamine dose equal to 80% of the total number of milligrams of heparin given resulted in no detectable plasma heparin in 23 of the 34 patients one-half hour after administration. No patient required protamine in an amount greater than the total number of milligrams in the heparin dose to achieve heparin neutralization. Modest postoperative chest tube drainage (mean, 784 ml in 48 hours) in these patients provides clinical support for low-dose protamine administration for heparin neutralization at the conclusion of cardiopulmonary bypass.