Archival paraffin sections from normal salivary gland tissue and salivary gland neoplasms were stained by immunoperoxidase technique with a well characterized cytokeratin antibody (PKK1). In normal parotid tissue, myoepithelial cells and peripheral cells of larger ducts were selectively stained. In pleomorphic adenomas, most cells were stained, the staining being somewhat stronger towards the duct lumina. In basal cell adenomas, only cells adjacent to the duct lumina were stained where a differentiation of cells into peripheral and ductal was seen. In adenolymphomas basal cells were stained, and in oncocytomas small elongated cells reacted with the PKK1 antibody. Only a few duct cells in an acinic cell carcinoma were reactive and in mucoepidermoid carcinoma, peripheral epidermoid cells were strongly stained. In adenoid cystic carcinoma, mostly duct cells were stained whereas the peripheral ones remained unstained. Although the intermediate filament protein expression is very stable during tumorigenesis, the staining with the presently used monoclonal antibody in salivary gland neoplasms differed markedly from what could be expected according to current views on the participation of this cell type. This supports our view that cells in tumors should be characterized on the basis of their staining, i.e. state of differentiation and not on their presumed histogenesis.