Mouse hybridomas secreting monoclonal antibodies against rabbit uterine progesterone receptor (PR) have been prepared. Several of these immunoglobulins exhibited high affinity towards human progesterone receptor and two (LET 126 and LET 64) were selected as giving the best immunoperoxidase staining of human progesterone target organs. Using the indirect peroxidase-antiperoxidase method of Sternberger, optimal conditions for demonstrating PR involved brief fixation of frozen sections with formaldehyde-containing fixatives, among them picric acid-paraformaldehyde. This method allowed us to detect the receptor in breast carcinoma epithelial cells, T47D cell line, and uterine endometrium and myometrium. No staining was observed in intestine and muscle. Specific staining for PR was confined to the nucleus, irrespective of the concentration of progesterone in the blood of the patient. In a preliminary study of 27 human breast cancers by the immunocytochemical method, the presence or absence of nuclear staining for PR correlated well with the concentration of cytosolic progesterone receptor determined by a steroid-binding assay on tumor extracts. Differences in the intensity and distribution of staining within a section were observed, suggesting heterogeneity of the PR content of breast cancer cells. In 19 tumors, the immunocytochemical method for PR localization was also used in combination with a slightly modified Abbott ER-ICA staining for estrogen receptor to compare the distribution of both receptors within the same biopsy on adjacent frozen sections. Various combinations of estrogen receptor and PR contents that have been determined by steroid-binding assay have also been detected by the double immunocytochemical assay.