In the normal salivary gland, the monoclonal antibody to keratin 8 immuno-morphologically identifies the epithelium that covers acini and ducts. The monoclonal antibodies to keratin 17 and vimentin detect normal myoepithelium. The two keratins are found both in the epithelioid and mesenchyma-like components of pleomorphic adenoma, suggesting a single epithelial nature of this tumor. In all the morphological components of the pleomorphic adenoma, there are cells that combine protein expression of intermediate filaments normally labelling different cell subpopulations. This fact provides support for the hypothesis that the pleomorphic adenoma originates from bipotent precursor cells. A particular phenotype of pleomorphic adenoma elements is realized under the control of the local density of cells and microenvironment, as a result of which the cells expressing vimentin predominate in the mesenchymal component, keratin 8 in the epithelial tubular one.