In this study erythrocyte membrane ATPase enzyme activity changes and hemolysis developed in the postoperative period after replacement of the same type of prosthetic heart valves were investigated in twenty patients. The findings were compared with the values obtained from ten healthy people. Hemolysis was not observed in the preoperative period of the patient group; but mild degree of hemolysis was detected in the fourth and the eighth months of the postoperative period. Although no hemolysis occured in the patient group at the preoperative period, a significant decrease was observed in each of three ATPase enzyme activities as compared to the control group. At the fourth and the eighth months of postoperative period, there were significant decreases in the ATPase enzyme activities with mild hemolysis when compared to preoperative period. These results indicated that the ATPase enzyme activities decreased in the patient group could not be directly related with hemolysis and some other factors might contribute to the loss enzyme activities.