This ultrastructural study is based on nine cases of adenoid cystic carcinoma arising from the salivary glands. Electron microscopically, the tumors consisted of five cell types: relatively differentiated intercalated duct cells, secretory cells, undifferentiated cells, cells of a squamous nature, and myoepithelial cells. Special attention was paid to the characteristic extracellular substances noted by light microscopic examination of these tumors. The hyaline material of the pseudocysts contained not only mucopolysaccharide granules but also fingerprint-like structures formed by replicated basement membrane. Some of these structures had a dense core, probably composed of mucopolysaccharide granules. Cell islands surrounded by elasticlike material also were found. The true lumina were usually filled with membranovesicular structures originating from fragments of the lining cells, mainly those with squamous differentiation. These membranovesicular structures included electron-dense rods, filamentous whorls, and structures displaying periodicity.