The tumor specificity of twelve different monoclonal antibodies (Mabs) against carcinoembryonic antigen (CEA) was assessed by immunohistochemistry. The Mabs had previously been classified into three specificity groups (I–III) on the basis of their reactivity with purified CEA-related antigens by ELISA. Mabs belonging to specificity group III (n = 4) did not cross-react with any CEA-related antigen, including normal cross-reactive antigen of 160 kD molecular weight (NCA-160 = meconium antigen). All Mabs, except one, gave positive immunohistochemical staining of 75–100% of individual tissue samples of colorectal carcinomas and gastric adenocarcinomas. However, when tested against different normal adult tissues, the Mabs displayed marked differences in reactivity. Group III Mabs stained normal colon epithelium, but not parenchymal cells in other organs or, with one exception, cells belonging to the granulocyte and/or macrophage series. Group I and II Mabs, in contrast, stained parenchymal cells in normal colon, submandibular salivary gland, placenta, and pancreas (group I Mab only). They also stained infiltrating and circulating granulocytes and/or macrophages. Lack of cross-reactivity with NCA-160 is the single-best criterion for selecting anti-CEA Mabs with a high degree of tumor specificity. To ensure tumor specificity, CEA-positive, NCA-160-negative Mabs should be checked by immunohistochemistry against cryostat sections of colorectal carcinoma, normal pancreas, submandibular salivary gland, spleen, and liver and for reactivity against circulating granulocy