The authors have used monoclonal estrophilin antibodies and the peroxidase antiperoxidase technique to characterize the distribution of estrogen receptor in the human vagina, uterus, and fallopian tubes. Exclusively nuclear localization of estrogen receptor was observed in epithelial cells, stromal cells of the lamina propria, and smooth muscle cells with both immunohistologic studies and immunoelectron-microscopy studies. The endometrium in various regions of the uterus (uterine isthmus, corpus, and fundus) was stained for estrogen receptor, with similar staining intensities in each of the respective cell types. There was no systematic regional variation in the staining intensity or distribution of cells with stained nuclei. The functionalis of the endometrium showed distinctive variation in the intensity of the staining for estrogen receptor with the menstrual cycle. The staining intensity of both endometrial epithelium and stroma was greatly reduced in the functionalis during the secretory phase. The vaginal epithelium did show some variation with the menstrual cycle, but it was much less than that observed in the functionalis of the endometrium. The basal cells of the vaginal stratified squamous epithelium, strongly stained during the proliferative phase, were less strongly stained during the secretory phase. No variation in the staining intensity for estrogen receptor was observed in different regions of the fallopian tube (isthmus, ampulla, and infundibulum), and the staining intensity varied only minimally with the menstrual cycle. The serosa of the female reproductive tract, connective tissues in the muscularis and in the vicinity of blood vessels, as well as neutrophils, eosinophils, mast cells, and lymphoid cells in the female genital tract were not stained for estrogen receptor.