In the intestinal mucosa, diamine oxidase (DAO) seems to be involved in a feed-back regulation mechanism for the termination of proliferation. Therefore, we studied the DAO activity in large bowel tumors and in the non-affected mucosa of these patients in comparison with patients having a normal large mucosa. The DAO activity in the tumor tissue itself was diminished by 85% as compared to the surrounding mucosa. Comparing the colonic mucosa of normal and tumor bearing individuals, the DAO activity in cancer patients was diminished by 22%, while it was elevated by 64% in patients with polyps (biphasic response of the DAO activity). Histologically proven hyperproliferative mucosal alterations were indicated by a reduced DAO activity with 75% sensitivity and 42% specificity. It remains open whether this limited specificity may indicate a more sensitive reaction of the DAO activity to proliferative mucosal alterations than the histological examinations.