A dual radioisotope labeling technique was utilized to assess red cell survival differences between cells processed by either a bubble oxygenator (eight patients) or membrane oxygenator (eight patients) in 16 patients undergoing cardiopulmonary bypass surgery. Cells processed by a bubble oxygenator consistently had a shortened survival. The 30-minute recovery of cells was not significantly different between oxygenators in contradiction to some previous studies using plasma hemoglobin as an indicator of hemolysis. The results of this investigation confirm previous studies that a membrane oxygenator provides a survival advantage to red cells during cardiopulmonary bypass surgery.