A monoclonal antibody to progesterone receptor, KD68, was used to localize this receptor protein in 31 surgical specimens of benign and malignant human prostate. Progesterone receptor was detected almost exclusively in stromal cells. The most striking finding was the periacinar arrangement of stained cells in some specimens of glandular BPH, sometimes associated with close apposition of stained cells to the basement membrane of the acinar epithelium. In both benign and malignant specimens scattered stained cells were observed in some areas of fibromuscular stroma. Except for occasional stained epithelial cells in one benign and one estrogen-treated malignant specimen, both benign and malignant epithelium were negative.