The epitope H contains an O-linked N-acetylglucosamine residue in a specific conformation and/or environment recognized by the monoclonal antibody H (mAbH). mAbH stains two bands with Mr x10(-3) of 209 and 62 in lysates of cultured rat astrocytes. In addition, in extracts of cultured MCF-7 breast carcinoma cell line cells it stains cytokeratin 8 and five polypeptides originating from Triton X-100-soluble (Mr x10(-3) of 232, 67 and 37) and from the Triton X-100-insoluble (Mr x10(-3) of 51 and 50) fractions, respectively. In our previous studies we used the mAbH to investigate by immunostaining the expression of the epitope H in normal human brains, human brains with a variety of lesions, astrocytic tumors, infiltrating ductal breast carcinomas, fibroadenomas, and mitochondria-rich normal, metaplastic and neoplastic cells. In order to gain further insight into the expression patterns of the epitope H in human tissues we used the mAbH to investigate the immunohistochemical expression of the epitope H in normal human endometrium, including 30 cases of proliferative endometrium, 30 cases of early secretory endometrium, 30 cases of mid secretory endometrium, 30 cases of late secretory endometrium and 30 cases of decidual tissues. The main results were the following: 1) The decidual stromal cells presented in all cases high cytoplasmic expression of the epitope H; 2) The pre-decidual stromal cells presented in all cases of late secretory endometrium significant cytoplasmic expression of the epitope H ranging from moderate to high expression; 3) The non pre-decidual stromal cells of the functional endometrial layer presented in all cases insignificant cytoplasmic expression of the epitope H ranging from null to low expression; 4) The stromal cells of the basal layer of the endometrium and decidua did not express the epitope H in any case; 5) The endometrial stromal granulocytes did not express the epitope H in any case and 6) The blood vessel wall cells (endothelial and smooth muscle) of the endometrium through the whole duration of the menstrual cycle and of the decidua presented high cytoplasmic expression of the epitope H. It is concluded that decidualized and pre-decidualized human normal endometrial stromal cells show increased expression of the O-linked N-acetylglucosamine containing epitope H compared to non-decidualized endometrial stromal cells. These findings suggest that the expression of the epitope H may be under positive progesteronic control in normal human endometrium. Further investigation of the antigens bearing the epitope H might help to gain further insight into the histophysiology and the pathology of human endometrium.