Sensitivity and value of the quantitative measurement of the isoenzyme creatine kinase-MB by means inhibitory antibodies were studied in a number of experiments. The results indicate that concentrations of more than 3 U/1 are abnormal: they were found only in patients with myocardial cell damage but not after intramuscular injection or physical exercise. Determining CK-MB apparently offers a better way of assessing low or moderate increases of total CK activity than, e.g., the CK/GOT ratio. The method should therefore be introduced into clinical diagnosis. But the question of whether CK-MB measurement can differentiate between myocardial infarction and reversible myocardial cell damage remains unanswered: it is not 100% specific for myocardial infarction.