In human subjects, apart from in the kidney, diamine oxidase occurs mainly in the gut. Therefore this enzyme can be used as an indicator of intestinal integrity. In biopsies of rectal mucosa the diamine oxidase activity was assayed in 55 patients, 41 having a histologically normal mucosa and 14 being diseased. The determinations of the enzymic activity were supervised by statistical quality control. In the unchanged rectal mucosa the diamine oxidase activity was 40 nmol/min X g on average. In 7 patients with rectal polyps the enzymic activity was significantly diminished in these benign tumors (mean = 7.7 nmol/min X g) apart from one, where it was elevated. A decrease in diamine oxidase activity was further observed in rectal carcinoma and ulcerative colitis. Whether the reduction of intestinal diamine oxidase activity accompanies premalignant or malignant states or whether it is a general sign of a disturbance of intestinal integrity remains questionable.