Human male genitourinary tract was processed for the immunohistochemical demonstration of IgG and IgM, in order to verify the presence and the source of these immunoglobulins. Only the epithelia of the prostate gland and urethra showed a marked IgM-immunopositivity, localized in the cytoplasm of apical cells. An intense immunoreactivity for IgG was observed in the apical cells of the vesical epithelium; a less intense immunoreactivity for IgG in the pelvic apical epithelial cells was found. Immunostaining in the ureteric epithelium was absent. Clusters of IgM- and IgG-positive immunocompetent cells were present in the subepithelial layers of the organs observed in our study. This study indicates that IgM, present in mucosal surfaces, derived from an active secretion, as well as secretory IgA, whereas most of the IgG present in the human seminal plasma were probably due to transudation process. Immunoreactivity for IgG in the vesical and pelvic epithelia could depend upon to virus-specific IgG antibodies produced by the mucosa. This in turn can contribute to the total antiviral activity in mucosal secretions.