Immunohistochemical staining was performed on normal, endometriotic, neoplastic endometrial tissues and various epithelial tissues of other organs using a monoclonal antibody, 5D4, which was raised originally against skeletal and corneal keratan sulfate. The results showed that this antibody exhibited strong and consistent reactivity to glandular epithelia found in normal and endometriotic tissues. In endometrial carcinomas, moderate reactivity was noted in well- to moderately differentiated adenocarcinoma. Other epithelial tissues examined showed almost no or faint reactivities. These results suggested that 5D4 would be a useful tool for identifying the endometrial glandular epithelia in histochemical studies.