The measurement of serum CK-MB isoenzyme is a very sensitive and specific indication of myocardial injury since only myocardium has substantial amounts of CK-MB. Serum CK-MB levels are most helpful clinically when the total creatine kinase is nonspecifically elevated, as with intramuscular injections, cardiac catheterization, stroke, noncardiac surgery and electric cardioversion. Elevations of serum CK-MB occurring in Duchenne's muscular dystrophy and other neuromuscular disorders may be due to the presence of abnormal regenerative skeletal muscle fibers, which are known to contain large amounts of CK-MB isoenzyme. These examples emphasize that under normal, nonregenerative conditions, elevations of serum CK-MB are rare in the absence of myocardial injury.