The diagnostic significance of enzyme and isoenzyme ratios in serum relies on their different distribution at various levels: from organs to subcellular structures. Factors which modify the ratios are the molecular properties of enzymes and isoenzymes and their turnover in the cells and in the extracellular space. These pathophysiological principles are considered with respect to the Szasz ratio CK/GOT and its application for the differential diagnosis of acute myocardial infarction.