Fifty-eight salivary gland pleomorphic adenomas and 5 normal salivary glands were studied immunohistochemically with respect to intermediate filaments (keratin, desmin, and vimentin), actin and S-100 protein to observe the cellular differentiation of these tumor cells. Normal myoepithelial cells showed positive immunostaining for actin, vimentin and S-100 protein. Pleomorphic adenomas expressed keratin, vimentin and S-100 protein to various degrees, but only a few tumor cells of pleomorphic adenoma revealed actin. The results indicate that the tumor cells of pleomorphic adenoma show a bipolar differentiation capability of both epithelial and mesenchymal origins, although normal myoepithelial cells show only mesenchymal characteristics. The findings also support previous reports using light and electron microscopy, and also contribute to more precise diagnosis and a better understanding of the histogenesis of this tumor.