Using an immunohistochemical method, carcinoembryonic antigen (CEA) was evaluated in bioptic specimens from 12 colorectal carcinomas, 32 neoplastic polyps and from 12 preparations of normal rectocolonic mucosa. CEA was demonstrated in 7/12 samples of normal mucosa, always localized on the apical pole of the columnar cells. In carcinomatous specimens CEA was found in 11/12 biopsies and showed a localization predominantly intracytoplasmatic or in the surrounding stroma. In 8/12 tumor specimens tested, the histologically normal mucosa bordering the carcinoma exhibited a slight staining for CEA on the apical surface of the glandular cells. Specimens from 27/32 polyps biopsied appeared to contain CEA on the apical pole of the glandular cells. These results confirm that CEA is a normal constituent of the human adult rectocolonic mucosa; the difference in CEA content between benign and malignant mucosa is very likely quantitative rather than qualitative. No indications on the prognosis of the polyps could be obtained from this study.