The intermediate filament protein types of normal choroid plexus and ependymal tissue and their putative tumors were investigated. In normal human choroid plexus tissue, but not in ependyma, keratin could be demonstrated immunohistochemically. By immunoblotting, keratins 8, 18, and 19 were found, but glial fibrillary acidic protein (GFAP) was absent. In mouse and rat, choroid plexus epithelium and ependymal lining cells were keratin-positive. In addition, many ependymal cells were vimentin-positive. Keratin was immunohistochemically found in three of four choroid plexus papillomas, two of two choroid plexus carcinomas, and the lining cells of three neuroepithelial cysts. GFAP-positive cells were present in some choroid plexus tumors. In contrast, none of the eight ependymomas contained keratin, but all were strongly positive for GFAP. The results show that choroid plexus lining cells and choroid plexus tumors have true epithelial characteristics in their cytoskeleton, in contrast to ependymomas, which do not show keratin positivity but show glial filaments, as would be seen in astrocytic tumors.