Acinic cell carcinomas, in some instances, contain a component of intercalated duct cells. However, the manner in which this element is integrated within the more obvious acinar cells, as well as the role neoplastic intercalated duct cells play in determining morphologic patterns in acinic cell tumors, has not been fully investigated. Ultrastructural study and immunostaining with antibodies to cytokeratins and to S-100 protein carried out in nine cases of parotid acinic cell carcinoma suggest two basic differentiation patterns. In three cases, the lesions were essentially composed of acinar cells (with variation in the number and form of secretory granules), and one of these tumors was unique in having ultrastructural evidence of differentiated myoepithelial cells. In the second group of six cases, there was light microscopic, ultrastructural, and immunohistochemical evidence of a significant component of intercalated duct cells. By means of both immunostaining (intercalated ducts were positive for keratin and S-100 protein; acinar cells were negative for both antigens) and electron microscopy, flattened-to-cuboidal intercalated duct cells were noted to enclose and, presumably, to be involved in the formation of microcystic spaces. Acinic cell carcinomas with a more solid growth pattern contained groups of intercalated duct cells positive for keratin and S-100 protein. Ultrastructurally, these cells were organized into well-formed ducts related to nests of acinar cells. Acinic cell carcinoma is another class of salivary gland tumor in which there can be an integrated proliferation of intercalated duct and acinar cells and, infrequently, of myoepithelial cells, all organized in a simulation of the intercalated duct-acinar unit of the normal salivary gland.