Monoclonal antiestrophilin antibodies (H226Sp gamma and H222Sp gamma) were used with an indirect immunoperoxidase technique to localize estrogen receptor in frozen sections of normal human endometrium. Estrogen receptor was detected in the nuclei of the vast majority of epithelial and stromal cells from all early, middle, and late proliferative phase endometria. However, middle and late secretory phase endometria showed a dramatic reduction in the amount of estrogen receptor localized. Specific staining for estrogen receptor in the functionalis of middle and late secretory phase endometria was weak and limited to nuclei of scattered epithelial and stromal cells. In contrast, strong staining for receptor was observed in epithelial cell nuclei of some glands in the basalis. The early secretory phase appeared to be a period of transition from the strong and ubiquitous staining for receptor characteristic of proliferative phase endometria to the weak, focal pattern of estrogen receptor localization characteristic of the functionalis from middle and late secretory endometria. Postmenopausal endometria showed consistently strong nuclear localization of estrogen receptor in epithelial and stromal cells. Myometrial smooth muscle cell nuclei also contained estrogen receptor. However, the endothelial cells of uterine vessels showed no localization of receptor. Specific staining for estrogen receptor was always limited to nuclei; no specific cytoplasmic staining was observed.