Carcinoembryonic antigen (CEA) mRNA expression was studied in 14 cases of normal colorectal mucosa and colonic adenocarcinomas using in situ hybridization with a 32P-labelled cDNA probe to the unique 3'-untranslated region of CEA. This approach has the advantage that the target mRNA remains in the cell of origin, whereas there is considerable ambiguity in immunocytochemistry data for CEA because the protein is secreted. Furthermore, the specific cDNA probe overcomes potential problems of immunological cross-reactivity with other members of the CEA family. The results demonstrated that abundant, heterogeneously distributed CEA mRNA was present in colorectal adenocarcinomas, with the highest levels in cells lining glandular structures. Parallel immunohistochemistry with anti-CEA monoclonal antibody A5B7 showed that the regions of tumours with the highest levels of CEA mRNA also had the highest CEA protein levels, suggesting that the heterogeneous distribution reflects CEA expression rather than differential secretion of the protein. In the normal colonic mucosa, CEA mRNA expression was observed in surface epithelial cells and goblet cells of the upper crypts, with very low hybridization in the mid crypt and at the base. This crypt-surface distribution was identical to that observed for CEA protein. In situ hybridization therefore confirms that high levels of CEA mRNA are expressed in differentiated surface epithelial cells of the normal colon.