Biotransformation or drug-metabolising enzymes have an important function in the detoxication of ingested toxic, carcinogenic, or tumour promoting compounds. Enzyme activity and isoenzyme composition of three biotransformation systems: glutathione S-transferase, uridine diphosphate-glucuronosyltransferase, and cytochrome P-450 were studied in normal small and large intestinal mucosa from three kidney donors. The activity of most drug-metabolising enzymes decreases slightly from proximal to distal small intestine, whereas in the mucosa of the large intestine a sharp fall in activity was observed. The isoenzyme composition for each of the three biotransformation systems changed from the small to the large intestine. Class Alpha glutathione S-transferases were not expressed in the colon, in contrast to the small intestine where both Alpha and Pi class isoenzymes are present. In addition, with monoclonal antibodies fewer protein bands for UDP-glucuronosyltransferases and cytochrome P-450 were detected in the colon. In the small intestine both isoforms P-450(4) and P-450(5) were present, whereas in the colon only reduced amounts of cytochrome P-450(4) could be visualised. For UDP-glucuronosyltransferase, 53 and 54 kDa proteins could be detected in the small intestine, but in the colon there was only weak staining of the 54 kDa band. In the normal human colon enzymes are less active and there are fewer isoenzymes present in the mucosa than in the small intestine. This implies a lower level of the detoxifying potential in the colon, which might be important in regard to the high rates of carcinogenesis in the colon.