Most of the creatine kinase (CK:EC 2.7.3.2) activity is present in skeletal muscle and myocardium. However, some activity in other organs has been reported. Theoretically, the destruction of organs that contain CK activity should release soluble enzymes into the general circulation. The effects of various destructive processes on serum creatine kinase isoenzymes in 70 patients were studied. The MB isoenzyme was demonstrated in 17 of 70 patients (24%), and the BB isoenzyme in 12 of 70 patients (17%). MB activity ranged from 3 to 12 U/l, or 0.7% to 10% of the total CK activity. BB activity ranged from 2.5 to 21 U/l, or 0.4% to 18.6% of the total CK activity. Interpretation of the results of CK isoenzyme studies should be made very carefully, especially if laboratories use CK-MB methods that measure MB and BB in combination. The diagnosis of myocardial injury should never be based solely on the presence of creatine kinase MB isoenzyme in the serum, but should be supported by additional clinical and laboratory data.