A series of rat and mouse monoclonal antibodies to the human progesterone receptor (PR) has recently been produced. These antibodies were used for the immunocytochemical identification of PR in several mammalian species including humans. The specificity of the monoclonal antibodies for PR was confirmed by using competition studies with purified PR and by comparison of the immunostained tissues, known from steroid binding assays to be receptor rich, with immunostained tissues known to be receptor-poor. Immunoreactive PR was found in the nuclei of uterine epithelial, stromal, and smooth muscle cells; benign ductal and lobular epithelial cells of the breast; ovarian surface epithelium; ovarian stroma and luteal cells; pulmonary parenchymal cells; and selected pituitary parenchymal cells. A proportion of the following selected human tumors expressed PR: breast carcinomas, endometrial carcinomas, ovarian carcinomas, and meningiomas. PR was localized to the nuclei of all progesterone target tissues even under conditions where the vast majority of the receptor is unoccupied by steroid, suggesting that the unoccupied as well as the steroid-occupied form of PR are predominantly nuclear proteins, as observed previously for estrogen receptor and rabbit PR.