The CK-2 and CK-3 isoenzymes of human serum creatine kinase (CK) can be further subdivided into five isoforms (subforms derived from the same isoenzyme). Three are derived from CK-3 and two from CK-2. The formation of these isoforms is a postsynthetic phenomenon brought about by a serum carboxypeptidase that acts on the M monomer of the enzyme. Sera from healthy subjects contain CK-3(1) as the dominant isoform with lesser amounts of CK-3(2) and CK-3(3). Following damage of muscle tissue, the serum isoform distribution changes as a result of the increased release of CK enzyme. This provides more diagnostic information concerning acute myocardial infarction and other muscle diseases than is available from routine CK isoenzyme analysis.