Mouse monoclonal anti-urine protein 1 antibody and the biotin-streptavidin-peroxidase technique were used for the immunohistochemical demonstration of human protein 1 in prostatic tissue of both sexes. In the female prostate (Skene's gland), like the male prostate, high expression of human protein 1 was observed on the luminal surface and in the apical cytoplasm of secretory cells of prostatic glands, as well as on the luminal surface of the epithelium of the large ducts of the female prostate and urethra. Expression was also found in the membranes of secretory and basal cells of the glands, in membranes of the urethral uroepithelium and of the female prostate ducts, in the content of glands and ducts, as well as in vascular endothelium and smooth muscle. Human protein 1 (urine protein 1) expression in the secretory cells of the male and female prostate and its incorporation into the surface of cells lining the lumina of the female urethroprostatic complex is indicative not only of the secretory role of protein 1 but also of its potential protective properties operative in shielding the uroepithelium from the aggressive urinary environment. All genito-urinary tissue, and especially the female prostate, were found to be a potential source of urine protein 1 (human protein 1), refuting the notion held so far that it is exclusively the genito-urinary prostatic tissue of the male that participates in its production. The corresponding immunohistochemical distribution of human protein 1 in the same structures of the male and female prostate provides yet another analogous functional-morphological parameter of prostatic tissue in both sexes and further evidence supporting the non-vestigial concept of the prostate in the female.