Diagnosis of injury to the myocardium is facilitated by information on the activities of creatine kinase (EC 2.7.3.2) MB isoenzyme (CK-MB) and lactate dehydrogenase (EC 1.1.1.27) isoenzyme 1 in serum, thee isoenzymes being present in higher activities in the myocardium than in other tissues or in normal serum. The temporal relationships of these isoenzymes, total creatine kinase, total lactate dehydrogenase, and aspartate aminotransferase (EC 2.6.1.1) are highly sensitive and specific for acute injury to the heart, particularly acute myocardial infarction. Chronic heart diseases, electric cardioversion for heart rhythm disturbances, coronary catheterization, and exercise usually do not produce increases of CK-MB, although abnormal aspartate aminotransferase, creatine kinase, lactate dehydrogenase, and lactate dehydrogenase isoenzyme 1 activities are seen in some individuals. Many other causes of increased activities of these enzymes and isoenzymes in serum are unrelated to injury to the heart. Because CK-MB is present in the skeletal muscle in low activities, substantial injury to skeletal muscle can increase CK-MB activities in the blood to abnormal values. Pulmonary embolism can mimic myocardial infarction in its clinical presentation. In patients with an accurately known time of onset of symptoms and serial enzyme analysis every 12 h during the first 48 h, acute myocardial infarction can be distinguished from pulmonary embolism by determinations of creatine kinase, CK-MB, aspartate aminotransferase, and lactate dehydrogenase isoenzyme 1 in serum.