In healthy human temporal bones, immunoglobulins (IgG, IgA) and the secretory component are demonstrated by two immunohistochemical methods in epithelial cells and in the lumen of the endolymphatic sac. The normal human inner ear does not otherwise show any localization of immunoglobulins. By means of indirect immunofluorescence, antibodies against healthy inner ear tissue were found in the serum of 15 out of 21 patients with double-sided progressive or fluctuating sensorineural hearing loss of undefined etiology. In 2 cases of Cogan's syndrome, it was also possible to demonstrate serum antibodies against epithelial structures of the cornea. The lymphocyte mitogen stimulation test performed with the fresh blood of 5 patients with positive detection of serum antibodies against healthy inner ear tissue was negative.