The distribution of lysozyme in normal gastric and duodenal mucosa was studied by light- and electron-microscopic immunocytochemical techniques (direct enzyme-labeled antibody method). In the duodenal mucosa, lysozyme was found in the Paneth cells and the epithelial cells of Brunner's glands. Electron-microscopically, lysozyme was found in rough endoplasmic reticulum and perinuclear spaces, which were assumed to be protein-synthesizing organelles, and also in the secretory granules of Paneth cells. Additionally, lysozyme was detected in the stomach in mucinous granules and in some parts of the rough endoplasmic reticulum within the epithelial cells of the pyloric glands, the mucous neck cells of the fundic glands, and in several surface epithelial cells of the plyoric and fundic regions. This suggests that some quantity of lysozyme in gastrointestinal secretion originates from the gastric and duodenal glands, and that it acts as a defense mechanism in the gastrointestinal tract.