The IgG1 monoclonal antibody, 44–3A6, was raised against the human lung adenocarcinoma cell line, A549. It has been shown to react with a 40,000 MW protein found on the cell surface, which is preserved in formalin-fixed paraffin-embedded tissues. A recent study of pulmonary carcinomas utilizing immunohistochemical methods showed exclusive binding to lung adenocarcinomas, subsets of neuroendocrine tumors, some carcinoids and a subset of large cell carcinomas. Reactivity was not seen in squamous cell carcinomas and small cell neuroendocrine carcinomas. In addition, melanomas, sarcomas and hematologic malignancies do not express this antigen. We now report on the reactivity pattern of 44–3A6 in adenocarcinomas of nonpulmonary primary sites and in normal adult organs. Strong diffuse staining of neoplastic cells in adenocarcinomas of the stomach, colon, pancreas, gallbladder and breast was noted. Adenocarcinomas arising in the endometrium, ovary, kidney, prostate, thyroid and liver were either negative or showed weak and/or focal reactivity. Strong staining patterns were even noted in adenocarcinomas which had an ‘undiffer-entiated’ component; i.e., lacking well-defined glandular elements.Immunoreactivity was noted in epithelial cells in several tissues from which these adenocarcinomas arose including the bronchial tract, stomach, small intestine, pancreas and colon, whereas epithelial cells from the endometrium, kidney, ovary, prostate and thyroid were negative or showed diffuse weak immunoreactivity. Our findings indicate that monoclonal antibody 44–3A6 recognizes an epithelial antigen on subsets of normal as well as transformed glandular epithelia. The differential pattern of expression of its target antigen probably reflects differences in tumor genesis and/or differ