The mucoepidermoid tumor of the salivary glands is relatively rare; its ultrastructure is analysed on the basis of four cases. Three secretory tumour cells can be differentiated: one tumour cell with mucous secretion and two tumour cells with abortive secretion which are different in structure. The dysregulated and abortive secretion is due to a defective functional differentiation and defective innervation of tumour cells. Further characteristic tumour cells without secretion are: epidermoid cells, myoepithelial cells and clear cells. The variety of the tumour cells in the tissue is due to the broad cytogenic prospection of acinar and tubular epithelium of the salivary glands. Recommendations are given for practical electron-microscopic diagnosis by means of typical leading elements of the structure. The author tries to assess the gravity of the mucoepidermoid tumour by ultramicroscopic findings. The tumour is classified as potentially malignant. Therapeutic aspects are discussed dependent upon this grading.