We examined the expression of carcinoembryonic antigen (CEA) and cytokeratin (CK) as well as the sialo- and sulphomucin content of 40 hyperplastic polyps (HPs), 6 mixed hyperplastic-adenomatous polyps, 30 adenomas and 40 adenocarcinomas of the colorectum. HPs showed a positive CEA expression in 95% of cases and a decreased sialo- and sulphomucin content compared with normal mucosa. Similar changes were observed in adenomas with low-grade dysplasia. The increase in CEA expression from HPs and adenomas to carcinomas was accompanied by a reduction of sialo- and sulphomucins with about three fourths of carcinomas being sialo- and sulphomucin negative. Oncofetal antigen expression concomitant with mucin changes observed in HPs may indicate impaired cellular maturation at a functional level before dysplastic changes become visible. CEA and CK positive macrophages were found in carcinomas predominantly at sites of tumor disruption and necrosis as well as within veins and lymphatic vessels. Our findings suggest that macrophages may play a role in CEA and CK release into the circulation and thus may be determinants of tumor marker serum levels.