This ultrastructural study, based on 12 cases of adenoid cystic carcinoma of the major and minor salivary glands, was conducted to determine the role and extent of participation of myoepithelial cells in their histogenesis. The tumors were composed of four major cell types; intercalated duct, myoepithelial, secretory, and pluripotential reserve/stem cells. The cellular composition of adenoid cystic carcinoma is similar to that in the "terminal tubule" complex stage of a developing salivary gland except that in the tumor the pluripotential reserve/stem cells differentiate predominantly along the intercalated duct cell line rather than secretory cells as in the acinic cell carcinoma. Furthermore, adenoid cystic carcinoma appears to contain a far greater number of myoepithelial cells than acinic cell carcinomas.