The degree of intraoperative myocardial injury was determined in two groups undergoing different methods of myocardial protection by observing the LDH and its isoenzymes up to the 11th postoperative day. Both groups consisted of patients who underwent mitral- and aortic-valve replacement. In the first group myocardial protection consisted of Kirsch cardioplegia (2) combined with conventional coronary perfusion in cases of aortic-valve replacement. In the second group the same initial cardioplegic procedure was followed by the noncorpuscular, hypothermic and aerobic coronary perfusion by Bleese and Döring (1). No difference was noted among the mitral valve patients. During the 11 days postoperative period, the aortic valve patients in the first group demonstrated a significantly higher LDH activity than the mitral-valve cases whereas LDH activity of aortic-valve patients in the second group resembled that of the mitral valve cases. Since the length of extracorporal circulation and ischemia did not vary significantly, the LDH behavior in the aortic-valve cases of the first group may be explained by the inferiority of the myocardial protection method used. In comparison with the other methods conventional coronary perfusion resulted in more myocardial injury.