1. The immature multilayered epithelium of the prostate glands and ducts reacts positively throughout the fetal period up to puberty for the following antigens: Acid prostate phosphatase (PAP), the prostate-specific antigen (PSA), keratins demonstrated by the broadly reacting keratin antibody "AE1 + AE3", keratins from stratum corneum, the keratins 7, 8, 18 and 19, binding sites for the peanut agglutinin (PNA) and carcinoembryonic antigen (CEA). The expression of PAP and PSA varies and can in individual cases even be nonexistent. Alpha-1-antichymotrypsin (ACT) is not demonstrable until birth. It is found only focally in two thirds of the cases. The prostate stroma proves positive for vimentin as well as desmin. In the prostate glands, vimentin may occur focally. Squamous cell metaplasias in the fetal prostate show positive for all antikeratin antisera as well as PNA and CEA. The epithelium of the ejaculatory ducts and seminal vesicles proves positive for all keratins investigated. Vimentin as well as PNA-binding sites are found on a general basis. The urothelium of the prostate urethra also contains all keratins and, in addition, PNA and CEA. Endocrine cells, positive for serotonin, are sparse in the prostate glands, in the prostatic ducts and in the urothelium. 2. The onset of the androgen effect during puberty leads very probably to the morphological differentiation of the immature glandular epithelium into the two cell types, basal cell and secretory cylindric epithelium which differ with regard to their immunohistochemical behavior. The glands of benign nodular hyperplasia, including special forms, as well as epithelial alterations with nuclear atypia behave immunohistochemically exactly like normal prostate glands. The markers PAP, PSA, CEA, ACT, the keratins 8 and 18 and focally occurring vimentin are expressed exclusively in the secretory epithelium of the prostate glands and ducts, and not in the basal cells. In contrast, antisera against the following antigens stain only the basal cells and basal cell hyperplasia: Keratins from stratum corneum, a common epitope of the keratins 4, 5 and 6 (antibody KA1) and both the estrogen and progesterone receptor. Squamous cell metaplasias behave immunohistochemically like the basal cells. The keratins 7 and 19, desmoplakin, PNA as well as the patients' own blood group antigens are found in both the basal cells and the secretory epithelium. Keratin 7 and the blood group antigens are always only found focally. PNA-binding sites are secreted by the cylindric epithelium. Atrophic glands and the glands in postatrophic hyperplasia share immunohistochemical features with both the basal cells and the secretory epithelium.(ABSTRACT TRUNCATED AT 400 WORDS)