The effect of gastrointestinal surgery on serum creatine kinase activity was studied in 30 patients. The MB isoenzyme was demonstrated in sera of 30% of the patients and BB isoenzyme in 23%. MB content varied from 0.8 to 10.3% of the total creatine kinase activity, and the BB content from 0.6 to 18.4%. The CK-BB was probably of gastrointestinal origin, since gastrointestinal tract contains high CK activity with BB isoenzyme predominating. A cardiac origin for the observed serum CK-MB isoenzyme increase would seem the most likely, although no patients showed evidence of electrocardiographic changes. Increased CK-MB activity has been observed in myocardial ischaemia without infarction.