Immunoglobulins G (IgG) and A (IgA) were found in plasma cells, osteocytes, chondrocytes and in connective tissue of active otosclerotic (= otospongiotic) lesions. The application of antibodies against paramyxovirus and rubella virus antigens reproducibly determined the expression of these antigens at different sites in the otosclerotic stapes, but also in the epithelial cells of the overlying middle ear mucosa. By using specific T-lymphocyte and B-lymphocyte markers, about 80% of the lymphocytes present in the otosclerotic footplate were revealed to be T-lymphocytes. There was neither deposition of immunoglobulins nor any expression of viral antigens in non-otosclerotic footplates which were investigated as controls.