Postoperative heparin rebound was investigated in 50 adult patients undergoing cardiopulmonary bypass with the use of the Hepcon heparin analyzer. Prior to bypass each patient received 2 mg/kg of heparin. During bypass, the activated clotting time (ACT) was utilized to assess the need for additional heparin to maintain the value between 300 and 400 seconds. The average amount of heparin given was 160 mg. Once cardiopulmonary bypass was terminated the Hepcon unit was employed to determine the actual amount of active circulating heparin and to calculate the dose of protamine sulfate. The average amount of protamine administered intraoperatively was 200 mg. The overall mean ratio of protamine-to-heparin was 1.25 : 1. Once hemostasis was achieved, no circulating heparin was measured with the Hepcon unit, and the ACT value had returned to its baseline, the incisions were closed and the patients were transferred to the intensive care unit. One hour later a blood sample was obtained and analyzed by the Hepcon unit for any heparin rebound. We found that 26 patients (52%) had circulating heparin and required an additional dose of protamine, averaging 70 mg. Drainage from the thoracotomy tubes averaged 400 cc in the first 24 hours, and a mean of 2 units of packed cells was infused. Three patients (6%) did not require any blood transfusions. The use of the Hepcon unit has produced a safe and expedient method of analyzing and neutralizing active circulating heparin in patients following cardiopulmonary bypass. It is a useful adjunct in blood conservation because it reduces excessive postoperative blood loss associated with heparin rebound.