Carcinoembryonic antigen (CEA) has been demonstrated in endocervical but not in endometrial adenocarcinomas, suggesting a valuable differentiating feature. Using an immunoperoxidase technique for CEA, alcian blue (AB)-periodic acid-Schiff stain for acidic and neutral mucosubstances, respectively, and Mayer's mucicarmine for epithelial mucosubstances, 13 endocervical and 21 endometrial adenocarcinomas were studied to delineate differentiating criteria. CEA was present in all endocervical adenocarcinomas with strongly positive immunostaining in 69%, diffusely throughout the tumor in 77%, intracellularly in 77%, in luminal secretions in 39%, and at the cell surface in 31%. Only 11 (52%) endometrial adenocarcinomas contained CEA, 82% with weak immunostaining, 64% focally, 82% at the cell surface and 36% intracellularly. Mucin was present in 12 (92%) endocervical lesions with AB-positive intracellular and luminal secretion in 83%, strong positivity in 42%, and a diffuse distribution in 60% with all three stains. Moderate or small amounts of mucin were present in 19 (91%) endometrial lesions, focally in 80%, in luminal secretions in 94%, and intracellularly in 42%. Thus, the majority of endocervical adenocarcinomas showed abundant diffusely distributed intracellular CEA and mucin, and mucinous luminal secretions. Half of the endometrial adenocarcinomas contained focal small amounts of apical surface CEA. Mucin was present focally particularly in luminal secretions.