Gastric mucus was stained with Azure A, a cationic dye, which had the highest affinity with macromolecular constituents of the mucus, under such conditions as 0.2% Azure A-0.5% NaHCO3 solution (pH 8.1) in dye concentration, staining for ten minutes, 37 degrees C in reaction temperature and the salt concentration and ionic strength below 6.0 x 10(-2). In rat and resected human stomachs, gastric mucus was clearly stained under these conditions. In human subjects, the in vivo stained muscu was observed endoscopically. The pyloric gland region. The difference was seen in the pattern of the gastric area between the fundic and pyloric gland region. Histological examination revealed that only the mucous layer was stained with Azure A. The stained macromolecules in the mucus and factors affecting the staining were discussed.