Normal and well differentiated neoplastic canine tissues were immunohistochemically stained for keratin, vimentin and desmin intermediate filament proteins using commercially available monoclonal antibodies. Keratin was detected in 56 of 57 carcinomas, vimentin in 59 of 62 sarcomas and desmin in three of four muscle cell tumors. Most normal and neoplastic tissues expressed only one type of intermediate filament; exceptions were one hemangiosarcoma and one pulmonary carcinoma in which there was coexpression of vimentin and keratin proteins. Since immunohistochemical detection of intermediate filaments has tissue-specific distribution in the majority of well differentiated canine neoplasms, these stains may be useful in the differential diagnosis of anaplastic canine tumors. However, the monoclonal antibodies to cytokeratin which were tested in this study failed to detect intermediate filaments in liver, pancreas and salivary glands which suggests that these antibodies may also be unable to detect epithelial tumors derived from these tissues. In addition, in nine neoplasms, the normal tissues adjacent to neoplastic cells failed to stain for the intermediate filament normally expressed. When this occurs, evaluation of intermediate filament expression is invalid for the determination of tissue of origin of the neoplastic cells.