The aim of the work was to define the distribution of phages administered per os to children for medical reasons, and the immune response. 102 children aged from 5 days to 15 years with different diseases of bacterial etiology (pneumonia, sepsis, urinary infection, pharyngitis/sinusitis, enteral infection) were monitored. Pyobacteriophage was being included into the complex therapy. The drug was administered per os. In 6/7 of blood, 48/55 urine and 64/75 stool samples taken on the 3-5th day of treatment different components of pyobacteriophage were revealed. The titers varied from 103 to 105 pfu/ml. No age differences were seen. In two weeks after the onset of the phagotherapy the antibodies to phages were tested in the blood serum using the neutralization reaction method. The blood samples were taken from 31 patients. In 14 of them the antibodies neutralizing 52.5-97.3% of the phage activity were seen. A significant age-related peculiarity was determined: in newborns and infants the antibodies were not revealed or their activity was low. Obtained results confirm the reasonability to use of peroral phagotherapy in gastro-intestinal infections. At the same time it was ascertained that the phages taken per os can permeate into the internal environment of the organism and thus the peroral phagotherapy can be used to treat systemic infections and urinary tract infections as well. Absence or low production of the antiphage antibodies in newborns and infants suggests high efficacy of the phagotherapy in this age group.