We have identified and described a distinctive type of cell which is characteristic of the "mixed" salivary tumour. This "hyaline cell" or plasmacytoid cell is particularly common and conspicuous in "mixed" tumours of the palate and other sites in the mouth. It occurs also in tumours of the major glands, but with much lesser frequency. The hyaline cell is found in "mixed" salivary-type tumours in other sites, e.g. the skin. It is not present in the other types of salivary tumour, notably adenoid cystic carcinoma, adenolymphoma, mucoepidermoid and acinic tumour. Ultrastructural study suggests that the hyaline cell is an indicator of myoepithelial differentiation. Current concepts of the acceptable pathways of myoepithelial differentiation in "mixed" tumours are discussed briefly. The specificity of the hyaline cell will probably prove valuable in separating "mixed" tumours from monomorphic adenomas, thus retaining the identity of the latter. The hyaline cell is almost as distinctive a feature of "mixed" tumours as is myxochondroid tissue and its specificity is of practical value in the diagnosis and classification of salivary tumours.