Determination of carcinoembryonic antigen levels in plasma (45 cases) and the immunohistochemical demonstration of tumor CEA (37 cases) were carried out in patients with endometrial adenocarcinoma. Twenty four out of 37 were also studied with the electron microscope. The plasma CEA level prior to therapy was significantly elevated (greater than 5.0ng/ml) in only one case (1/45:2.2%) of endometrial adenocarcinoma. CEA levels were more consistently elevated (4/17:23.5%) in patients with cervical adenocarcinoma. Immunoperoxidase staining of CEA (PAP method) was carried out using formalin-fixed paraffin embedded sections of 37 tumors. Tissue CEA activities were found in 9 out of 37 endometrial adenocarcinomas (24.3%) and 19 of 23 cervical adenocarcinomas (82.6%). Immunoreactive CEA was present in a high concentration on the cell surface of endometrial glands and less dense in their cytoplasm. In 9 of CEA positive endometrial carcinoma tissues, squamous metaplastic lesions were found in 4 cases and mucinous metaplasia in one case which were all CEA positive. With regard to the histopathological grade, CEA positive specimens were categorized G1 (4) and G2 (5) and all of the G3 were CEA negative. Seven of 9 cases of CEA positive specimens were examined under the electron microscope. There was no definite tendency in their ultrastructural characteristics, but all of the specimens examined revealed abundant cytoplasmic organelles suggestive of intracytoplasmic differentiation analogous to those of endometrial cells in proliferative phase. Moreover, fine structures of squamous and mucinous metaplastic cells were also described in detail.