The peptic ulcer is observed among the Transbaikalian Buryats 3 to 4 times less frequently than among the Russians living in the same environments. The national difference in the frequency of this disease is determined genetically. Revealing its cause is the most perspective way to determine the mechanism of the hereditary predisposition to the peptic ulcer. For this purpose, the Buryats and the Russians were investigated for peculiarities of fucoglycoproteids (FGP), the main chemical protective agents of gastroduodenal system. The blood group, the presence of antigenic properties of ABH(0) system, fucosuria as the intensity index of the secretion have been determined. It was found that the Buryats have the A (II) blood group less frequently than the Russians. Inability to secrete the "A" group FGP (which prevents the peptic ulcer development) into the mucus was observed more frequently for the Buryats. the quantity of the fucose discharged by the Buryats in the urine is twice as much as that discharged by the Russians. The results obtained suggest that the rare occurrence of the peptic ulcer among the Buryats is due to the excessive formation of FGPs, which compensates the defects of quality.