Although human labial gland secretions contain serous components such as the bactericidal enzyme, lysozyme, the presence of serous cells in this gland has yet to be clearly visualized under the electron microscope. The present study identifies lysozyme-expressing cells of the labial glands using microwave-fixed, Epon-Araldite-embedded specimens, which showed excellent preservation of both ultrastructural detail and antigenicity for post-embedding immunogold labeling of lysozyme. Ultrastructurally, all of the secretory cells of the glands appeared to be a mucous-type and have a serial maturation relationship, consistent with a previous report by TANDLER et al. (1969a): their secretory granules were electron-lucent and exhibited reactivity for mucus staining by the periodic acid-thiocarbohydrazide-silver proteinate (PA-TCH-SP) method. We classified them into two immature types (I, II) and two mature types (I, II). Their distinctive features were the following: 1) relatively small (0.5-1 microm) secretory granules and well-developed basal rough endoplasmic reticulum for the immature types; 2) larger (1-2 microm) secretory granules and well-developed Golgi apparatus, which showed intense PA-TCH-SP reactivity in the 2-3 trans-cisterns, for the mature types; 3) few secretory granules in the immature type I; and 4) the darkest appearance for the mature type II. Immunogold labeling with anti-lysozyme showed specific labeling of the two immature-type cells, in which gold particles were found mainly over the secretory granules and Golgi apparatus, and moderately over the rough endoplasmic reticulum. In the secretory granules, the labeling was distributed throughout the contents and was present even if they showed strong PA-TCH-SP reactivity; in the Golgi area, it was seen over the stacked cisternae, trans-Golgi networks, and condensing vacuoles. No specific labeling was seen in the mature-type cells or in the duct cells. These immature- and mature-type cells were almost equivalent to the "serous demilune or acinus" and "mucous tubule" cells, respectively, at the light-microscopic level. These results indicate that the traditional "immature mucous-type cells" of the human labial glands produce lysozyme and should be classified as seromucous cells.