The effects of prostacyclin on whole blood platelet count, blood coagulation factors, and postoperative bleeding were investigated in 20 patients undergoing aorta-coronary bypass. Eleven patients received heparin 2 mg/kg and prostacyclin 50 ng/kg/min during cardiopulmonary bypass (CPB). Nine patients received only heparin 3 mg/kg. CPB was by roller pump and bubble oxygenator primed with Ringer's acetate. Hypothermia to 28 degrees C was induced. In the control group, platelet count, corrected for hemodilution, was 70% +/- 15% (mean +/- SD) of pre-CPB value after 30 minutes of bypass and remained at this level 1 hour after CPB. In the prostacyclin group, the platelet count after 30 minutes was 85% +/- 17%, after 120 minutes 111% +/- 20%, and 1 hour after CPB 92% +/- 17%. There was a significant difference between the groups (p less than 0.05) from 60 minutes of CPB up to 1 hour after CPB. Prostacyclin allowed reduction of the heparin dosage while retaining anticoagulation as measured by activated coagulation time (ACT), fibrinopeptide A, and fibrinogen determinations. The ACT was more than 900 seconds in the prostacyclin group after 30 minutes of CPB, as compared to 523 +/- 118 seconds (p less than 0.05) in the control group. This difference diminished later during CPB. In the prostacyclin group, arterial blood pressure was 30 mm Hg or less during the first hour of CPB and the systemic vascular resistance was half of that in the control group. All patients survived. There were no clinical signs of neurologic damage. Postoperative bleeding was 352 +/- 61 ml in the prostacyclin group and 550 +/- 338 ml (NS) in the control group.