Total creatine kinase (CK) activity and isoenzyme distribution were determined in normal and tumour of gastrointestinal (GI) tract. Total CK activity per gram wet tissue appeared to be markedly reduced in malignant tumour tissue, yet it was raised in benign tumour tissue. CK-BB was the predominant isoenzyme in both normal and tumour tissue of GI tract. Alteration of isoenzyme pattern was noted between the normal and tumour tissue. In one patient, macro-CK was found in tissue homogenate.