In human seminal plasma it is possible to measure the levels of IgG and IgA, while IgM has been detected in trace amounts in only a few subjects. The source of these antibodies is unknown, although, under certain conditions, they seem to be produced in the sex tissues. Male human genital tract was processed for the immunohistochemical demonstration of IgG and IgM to verify the presence and the source of these antibodies. Only the epithelia of the prostate gland and urethra showed positivity to the immunohistochemical reaction, indicating the presence of IgM-secreting epithelial cells. Clusters of IgM- and IgG-positive immunocompetent cells were also observed in the subepithelial layers of all organs studied. The findings suggest that IgM are present in mucosal surfaces as a result of active secretion, as well as secretory IgA, whereas most of the IgG present in the human seminal plasma are probably derived by transudation process.