Murine monoclonal antibody (MAb) B72.3, prepared against a membrane-enriched extract of metastatic carcinoma and reactive with a high-molecular-weight determinant, designated tumor-associated glycoprotein (TAG)-72, was shown to be reactive immunohistochemically with 97% of a variety of primary adenocarcinomas of the stomach (n = 40). All "early" gastric carcinomas were reactive with MAb B72.3, although the average percentage cellular reactivity was lower than in "advanced" carcinomas. TAG-72 antigen was detected in benign lesions (i.e. adenomatous polyps and hyperplastic polyps) with intestinal metaplasia. Dysplastic lesions characterized by cellular atypia, abnormal differentiation, and disorganized mucosal architecture demonstrated higher TAG-72 expression than non-dysplastic epithelia. In contrast, normal gastric mucosa was generally non-reactive with MAb B72.3. Assays using serial sections of normal, benign and malignant gastric tissues with two MAbs (B1.1 and COL-6) directed against distinct epitopes of carcinoembryonic antigen (CEA) demonstrated differential reactivity, confirming that TAG-72 and CEA are distinct, non-coordinately expressed antigens. Our results suggest that TAG-72 antigen may be expressed in malignant and dysplastic epithelial cells, as well as in intestinalized epithelium of the stomach which has been closely related to subsequent carcinoma development. Hence, MAb B72.3 may be a useful immunohistochemical adjunct for detecting early foci of adenocarcinomas and premalignant lesions of the stomach.