Six pleomorphic adenomas were studied histochemically by electron microscopy after staining with ruthenium red, high iron diamine-thiocarbohydrazide-silver proteinate (HID-TCH-SP), and tannic acid-ferric chloride (TA-FeCl3); the latter two are quite specific for sulfated glycoconjugates. With ruthenium red staining, the proteoglycans of the myxochondroid stroma could be visualized as numerous extracellular 25 to 50-nm polygonal matrix granules with fine projecting filaments. Similar positive ruthenium red-stained intracellular granules were observed within the Golgi-derived vacuoles of the modified "myoepithelial" cells in the myxochondroid region. Secretion of these granules by exocytosis was occasionally observed. The modified myoepithelial cell in the myxoid region exhibited strong HID-TCH-SP- and TA-FeCl3-reactive sites within the Golgi-derived vacuoles. The intercellular space or microcyst of the cellular clusters and the extracellular matrix granules stained strongly positive with both HID-TCH-SP and TA-FeCl3. Some cells located at the periphery of the cellular clusters also showed similar but weaker reactive sites. No staining was obtained in neoplastic ductal epithelium. The above cytochemical observations indicate that the modified "myoepithelial" cell probably derives from the periphery of the cellular clusters and is the cellular source of the stromal matrix. Moreover, the observations are compatible with the histogenetic model recently proposed by Dardick et al. that the neoplastic, modified myoepithelial cell is the principal proliferating cell in pleomorphic adenoma.