In an immunohistochemical study, monoclonal and polyclonal antibodies have been used to identify cells and structures in the extraosseous part of endolymphatic sacs (ES) which were removed at autopsy from 30 persons. Intraluminal, intraepithelial, intravascular and perisaccular cells expressed the leukocyte common antigen. Immunostaining with antibodies against the CD4 and CD8 antigens revealed the predominance of CD4-positive T lymphocytes in the ES. A few lympoid cells were found to express the major histocompatibility antigen class II. Interdigitating cells of the Langerhans type were rarely found in the epithelial layer. B lymphocytes were present in the lumen and the stroma of the ES and IgA-or IgG-containing cells in the stroma only. IgA, secretory component and the J chain were detected within epithelial cells and in the lumen of the ES. Macrophages were observed in the lumen and the stroma. Our findings are in accordance with previously published data in animals and man and give further evidence of an important role of the ES in inner ear immune defence.