No one has counted the enzymes in the human organism, and probably their profuse number can only be estimated from the restrictions the finite mass of genetic material imposes. Still, nu­ merous catalytic activities have been identified and investigated for their clinical relevance. After one century of such research, surprisingly, only a handful of enzymes has survived the stern trial of clinical application to find regular medical use. Dis­ regarding genetic defects, most of the diagnostically used en­ zymes participate in metabolic conversions fundamental to life: the utilization, transfer or fixation of vital nutrients or of cheJ,llical energy. Creatine kinase is central to the function of the mitochond­ rion, the cellular power generator. The wider distribution of this enzyme is linked to tissues whose physiology crucially relies on an adaptable supply of energy, notably skeletal muscle, the brain, and the heart. Just as these three tissues vary in their pre­ ference for different energy sources, each produces a different isoenzyme of creatine kinase. Accordingly, the finding of elevat­ ed creatine kinase activity in blood can lead to a variety of interpretations, but in practice the diagnosis of acute myocardial infarction is the major consideration.