Morphological electron-microscopic evaluation of gastroduodenal mucosa in subjects who worked in the zone of radionuclide contamination after the Chernobyl accident in 1986-1987 and therefore exposed to low-dose ionizing radiation (18, 62 sGy on the average) discovered deficiency of tissue immunocompetent cells, dystrophic changes in the epithelium and stroma. In epithelial cells there were structural shifts in microvillae, organoids and nuclei. Epithelium, stroma and endothelium of blood vessels contained dark cells with thick hyaloplasm.