The Thomsen-Friedenreich antigen (T-antigen) Gal beta 1-3Ga1NAc, is a cryptic disaccharide structure on human erythrocytes. It is masked by sialic acid and uncovered by sialidases as neuraminidase. T-antigen is a tumor-associated antigen in epithelial tissues. This study reports preliminary immunohistochemical findings on the expression of the blood group related T-antigen in human endometrium. Dewaxed sections of 10 proliferative-, 6 interval-, 14 secretory endometrium, and 16 endometrial carcinomas, either untreated or pretreated with neuraminidase, were subjected to an indirect immunoperoxidase technique using the monoclonal antibody 49H.8. The monoclonal antibody 49H.8 did not bind to normal cyclic endometrium, unless pretreated with neuraminidase. Neuraminidase treatment revealed staining of apical membranes of glandular- and surface epithelium in early or manifest secretory endometrium only. Thirteen of sixteen carcinomas stained without neuraminidase treatment, while all carcinomas stained after pretreatment with neuraminidase. Staining was more widespread than in untreated carcinomas, resembling that of secretory endometrium. Our results suggest: 1) That the sialylated 49H.8 antigen seems to be a marker of cellular differentiation in the endometrium, 2) that the antigen defined by the monoclonal antibody 49H.8 might be a tumor-associated antigen in endometrial epithelial tissue.