This report is concerned with basic studies on chromo-endoscopic findings using Brilliant Blue (BB), with special reference to the affinity of BB dye with mucus and fibrin and also to the dissecting microscopic surface structure of the normal and cancerous gastric mucosa. The BB solution stained mucus and fibrin on the mucosal surface of the stomach at pHs ranging from 1 to 4. Stained mucus appeared translucent, and stained fibrin opaque. The mucus or fibrin clot bound with BB appeared blue at any pH of the dyebath, even with a green BB solution at pH 1. A thin layer of mucus was stained with BB and destained with a 1.0% sodium bicarbonate solution in a few seconds at 37℃, while a small piece of fibrin clot was stained and destained much more slowly (in approximately 30 minutes) under the same conditions. The normal mucosa (including fundic mucosa, pyloric mucosa, and that with glandular atrophy and intestinal metaplasia) was covered with a thin layer of viscous mucus. The chromo-endoscopic appearance depended mostly on the stain characteristics of BB and the fine properties of the mucosal surface of the stomach. The "well-demarcated redness" characteristic of a gastric cancer lesion mainly of the depressed type proved to reflect an unstained or less-stained mucosal territory associated with such factors as lack of the normal-appearing layer of viscous mucus over the cancerous surface, easy loss of the insoluble mucus- and fibrin-like substances attached to the cancerous surface, and compactness of the surface structure in the case of well-differentiated tubular adenocarcinomas.