The role of gastric acidity in regulating the bacterium composition in the small intestine was studied. Gastric acidity was determined with the Kay test. Intestinal juice obtained by a probe was examined for the presence and number of microorganisms. Parallel with the increase in gastric acidity, the number of bacteria in the jejunal juice generally decreased. Their composition also varied: in anacid patients E. coli predominated whereas in those with free hydrochloric acid, Streptococcus alpha haemolyticus and anaerobic Peptostreptococcus formed the bulk of the microflora. These data underline the importance of the role of acidity in the development of bacterial contamination in the jejunum.