The expression of mucin MUCZ was investigated in normal colonic tissue, in colonic adenomas and in carcinomas of the mucinous and non-mucinous type. The latter were subdivided into carcinomas originating from the adenoma-carcinoma sequence (ACS) and de novo (DN) carcinomas. The expression was assayed by immunohistochemistry with the monoclonal anti-MUC2 antibody CCP58 and by mRNA semiquantitation. MUCZ protein epitope CCP58 was strongly expressed in 2 I O h of normal colonic tissues, in 40% of villous and in 48% of tubular adenomas. Mucinous carcinomas exhibited strong expression in 72%, ACS carcinomas in 2 I Yo and DN adenocarcinomas in none of the tumors investigated. Compared with the adjacent nonmalignant tissue (transitional mucosa), CCP58 epitope expression in the tumor was higher in 74% of mucinous carcinomas, but equal or lower in 69% of ACS carcinomas and in 100% of de novo carcinomas. The alterations of MUC2 expression detected by immunohistochemistry in adenocarcinomas were confirmed on mRNA level. These data indicate that the MUCZ expression pattern is different in the 3 carcinoma types investigated. MUCZ over-expression occurs in the adenomatous tissue. It is atways maintained in mucinous carcinomas, but frequently decreased in non-mucinous ACS carcinomas. DN carcinomas are most frequently associated with decreased expression of MUC2.