Beta microseminoprotein (beta inhibin, PSP94), an unglycosylated protein of 94 amino acids with unknown function, is one of the predominating proteins in the secretion of the human prostate gland. In this work the authors have demonstrated that the expression of beta microseminoprotein is not restricted to the prostate and that the protein has a previously unrecognized widespread occurrence in the human body. According to radioimmunoassay, beta microseminoprotein immunoreactivity is present in many nonprostatic body fluids. The highest concentrations were found in secretions from the respiratory tract; in tracheobronchial fluid sometimes even at concentrations comparable to that in seminal plasma (about 1 g/l). Intermediate concentrations were found in gastric juice and some samples of secretion from the uterine cervix, whereas tears, saliva, pancreatic juice, bile, and mucus from the colon had low concentrations. According to gel chromatography, the molecular size of the beta microseminoprotein immunoreactivity present in tracheal fluid, gastric juice, and secretion from the uterine cervix did not differ from that of beta microseminoprotein in seminal plasma. The beta microseminoprotein immunoreactive component present in gastric juice had the same amino-terminal amino acid sequence as prostatic beta microseminoprotein (14 residues identified in material purified from gastric juice), providing further evidence for chemical identity of a nonprostatic beta microseminoprotein with the prostatic protein. Immunohistochemical staining with affinity-purified antibodies demonstrated the presence of beta microseminoprotein in many tissues, including the goblet cells in the tracheobronchial epithelium, tracheobronchial submucosal glands, certain mucosal cells in the antrum of the stomach, some glands of Brunner in the duodenum, and in parts of the mucosa of the colon. At least in the respiratory tract, the staining was localized to mucus-containing cells. beta microseminoprotein immunoreactivity also was localized to the cilia of the ciliated epithelium in the respiratory tract, the fallopian tubes, and the Gartner ducts of the uterine cervix. The pattern of tissue distribution of beta microseminoprotein found in this work indicates a connection of beta microseminoprotein with mucous secretions.