Six parotid pleomorphic adenomas were studied histochemically by electron microscopy after staining with ruthenium red. It was found that the myxoid region was filled with ruthenium red granules. Similar positive ruthenium red-stained intracellular granules were observed within the Golgi-derived vacuoles of the neoplastic myoepithelial cells in the myxoid region. Some cells located at the periphery of the double-layer ductal structure had the features of myoepithelial cell, they also showed similar positive ruthenium red-stained granules in the intracellular vacuoles and intercellular space. No ruthenium red-stained granules was obtained in neoplastic ductal epithelium and the mucus in ductal cavities were negative for ruthenium red stain. The observations indicate that the neoplastic myoepithelial cells in the myxoid region probably derive from the periphery of the double-layer ductal structure, they secrete the proteoglycans forming the myxoid region and play an important role in the histogenesis of pleomorphic adenoma.