Ultrastructural examination of 6 mucoepidermoid carcinomas (4 from parotid salivary gland and 2 from lung) was used to investigate tumor cell types, their interrelationship, histogenetic aspects and factors involved in the multiplicity of histological patterns in this tumor. On the basis of light-microscopic cellular features, mucoepidermoid carcinomas are generally thought to be formed by 3 principal cell types, mucinous, squamous and intermediate. However, low-magnification electron micrographs reveal that there are 2 basic tumor-cell types interpreted as modified luminal epithelial and myoepithelial cells. Both cell types may independently show a range of fine structural modification and organizational arrangement. Luminal epithelial cells have varying degrees of mucin production and lumen formation in some regions, while modification of these cells is one mechanism for squamoid and frankly squamous appearances in other areas. Preferential proliferation of the second type of tumor cell aligned on the outer aspect of luminal epithelial cells is responsible for the development of isomorphic regions composed of intermediate cells. Squamous metaplasia can also occur in relationship to these latter cells. Based on these observations and recent ultrastructural and histochemical studies of salivary gland tumors, it is suggested that, in addition to the excretory duct, neoplastically altered cells in the acinar-intercalated duct region must be considered as a potential source for the development of mucoepidermoid carcinomas.