Forty-three tumors were investigated by means of immunofluorescence with the use of antibodies against the following different classes of intermediate-sized (10 nm) filament proteins: 1) cytokeratins, 2) vimentin, and 3) desmin. In general, the immunologic features of tumor-cell intermediate filaments are those present in their tissue of origin. It can be seen, therefore, that, during neoplastic transformation, there are no major changes in the synthesis of the type of intermediate filament proteins when compared to normal tissues. Immunologic identification of these proteins furnishes the surgical pathologist with a quick and clear-cut way to differentiate tumors of mesenchymal origin from epithelial neoplasms, and in particular to distinguish between malignant lymphomas and lymph node metastases of undifferentiated carcinomas.