Creatine phosphokinase-MB (CPK-MB) is the most sensitive and the most specific indicator available for the diagnosis of an acute myocardial infarction. With the exception of after-cardiac surgical procedures, the degree and the duration of CPK-MB elevation in serum approximates the extent of an acute myocardial infarction, although a variety of factors may affect the reliability of such an index. Differences in the fractionation and assay methods for the creatine phosphokinase isoenzymes have produced conflicting documentation as to the presence of CPK-MB in tissues other than myocardium and the release of CPK-MB under conditions other than an acute myocardial infarction. The embryological development of the CPK-MB isoenzymes, as well as the various conditions involving increased CPK-BB serum activity, also deserve attention.