16 spontaneous tumors of the peripheral nerves and 18 spontaneous tumors of mesenchymal origin in BDVI rats were studied by peroxidase-antiperoxidase method using anti-serum (DAKOPATT) against protein S-100. The majority of spontaneous peripheral nerve tumors were of cystic histological structure identical to that of cystic neurinomas induced in rats by ethylnitrosourea and almost all of these tumors were S-100 protein positive. The incidence of spontaneous neurinomas in BDVI rats was in some experiments as high as 5%. All tumors of mesenchymal origin (except one lipoma) were S-100 protein negative: 2 fibromas, 6 fibrosarcomas, 3 malignant fibrous histiocytomas, one rhabdomyosarcoma and one hemangioendothelioma. S-100 protein is found, as in human pathology, useful for distinguishing tumors of schwann cell and mesenchymal origin in rats.