The morphology of the labial salivary glands in subjects with and without cystic fibrosis has been studied by light and electron microscopy. The terms ;light' cell, ;dark' cell, ;clear' cell, and ;myoepithelial' cell are used to describe the four cell types distinguished at the level of the electron microscope. The dark cells were found to stain specifically with alcian blue at and above pH 1.0, and the light cells to stain specifically using the periodic acid-Schiff (PAS) technique. This staining reaction suggests that an acidic mucosubstance is being produced by the dark cells and a neutral mucosubstance by the light cells. While no marked morphological differences were observed in the labial glands of patients with cystic fibrosis when contrasted with normal controls, it was observed that the cells from patients with cystic fibrosis contained quantitatively more mucus.