The results of a comparative histological study of the morphological reaction of parenchymal organs and organs of the immune system in broiler chickens using antibiotics and probiotics showed a positive effect of the probiotic drug on the body of broiler chickens of the experimental group. Two groups were formed according to the principle of analogues, a control and an experimental group of 35 heads each. Chickens of the control and experimental groups received the antibiotic enrofloxacin twice for 3 days up to 14 days of age according to the scheme of prevention of infectious diseases. Chickens of the experimental group received a feed additive (probiotic) based on microorganisms Bifidobacterium animalis from day 15 to 30. In a comparative study of the thymus of broiler chickens of the experimental group, the predominance of the cortical substance over the cerebral substance was noted in most of the lobules located on the periphery, and only in the merged lobules of the middle part of the thymus, the brain zone with a significant number of Ghassal bodies prevailed, a pseudoeosinophilic reaction was noted in the brain zone of the experimental chickens, as a result of the utilization of deposits and purification of the organ from the cells of the dead by the type of apoptosis. Histological studies of the spleen revealed activation of lymphoid follicles: in the chickens of the control group they were single on the preparation, and in the chickens of the experimental group there were three or more of them in one field of view. This indicates the activation of humoral immunity in chickens of the experimental group.In the study of the pancreas in chickens of the experimental group, a positive tendency was noted to increase the number and size of the islets of Langerhans, which regulate the function of carbohydrate metabolism.Analysis of the results of a comparative histological study of the morphological reaction of parenchymal organs and organs of the immune system showed that a number of reversible and irreversible pathological changes in organs were noted in the control group. Whereas in the experimental group where the probiotic was used, the organs corresponded to the physiological norm, which indicates a positive effect on the organs and on the body as a whole.