Morphologically unchanged mucosa of the fundal and pyloric segments of the stomach and duodenum was studied in 40 persons. In both the fundal and pyloric segments, superficial epithelium contained single interepithelial lymphocytes, their number was 52 +/- 5 and 59 +/- 4%, respectively. In the fossal and cervical glandular epithelium their number was nearly the same. In the villous epithelium of the duodenum the number of the interepithelial lymphocytes was 181 +/- 16, and in the criptal epithelium--78 +/- 14 per 1,000 enterocytes. The essential prevalence of the interepithelial lymphocytes in the duodenal villi, comparing to the gastric superficial epithelium is connected with the fact that the main function of the small intestine is absorbtion of various substances (including antigens). The presence of the interepithelial lymphocytes in the generative zones of the stomach and in those of the duodenum makes it possible to suggest that lymphocytes participate also in ensuring a normal cellular renewal of these organs.