The significance of creatine kinase (CK) in the diagnosis of acute myocardial infarction was evaluated. The serum level of CK-MB, a CK isozyme was determined by the immunoinhibition method. The CK-MB activity could be determined by the immunoinhibition method in a short time with an autoanalyzer, suggesting that the immunoinhibition technique is adequate as a method of emergency examination. The immunoinhibition method has the disadvantage of error induction because of contamination with CK-BB, CKm and macro-CK by this method. Thus, CK-MB activity should be determined in parallel with total CK activity. When a significant contribution from CK-BB is suspected, its presence should be confirmed by other methods. The present results suggest that the efficiency of coronary artery reperfusion therapy can be evaluated earlier by determination of CK-MB activity than by that of total CK activity.