We have explored the merit of a simultaneous study of sialomucin content and carcinoembryonic antigen (CEA) expression in the identification of early malignancy in adenomas. One hundred and thirteen colorectal adenomas were investigated by histochemical and immunocytochemical techniques. We compared adenomas from 'high risk' patients having synchronous carcinoma and 'low risk' groups with incidental polyps only. Twenty-three metaplastic and eight inflammatory polyps were also included. Our data suggest that size and dysplasia are not always closely related and that synchronous adenomas seem to carry a higher malignant potential than incidental polyps irrespective of size. The degree of O-acylation of sialic acids appears to be a sensitive indicator of early malignant change: loss of O-acylation was seen in all II adenomas with highly atypical foci ('focal carcinoma') but noted in only four of the remaining polyps with lower grade dysplasia (P less than 0.005). By contrast the intensity of staining for CEA was a gradual phenomenon and showed no statistically significant increase with the onset of malignancy. Inflammatory polyps showed staining characteristics similar to normal mucosa. Metaplastic polyps, however, revealed increased expression of CEA and reduced O-acylation with increased size which may reflect a disorder of growth and differentiation. Finally, by comparing these profiles of staining with those of normal mucosa, 'transitional' mucosa adjacent to carcinoma and carcinoma, we further illustrate a progression of changes occurring in colonic mucosa in carcinogenesis.