The effect of the probiotic Bacillus subtilis VKM B3701D and enrofloxacin on the functional state of the liver of egg-laying Highsex brown chickens was studied. To study the effect of a probiotic feed supplement on the physiological and biochemical status of chickens, egg-laying chickens were fed the Bacillus subtilis VKM B3701D bacterial strain at the vivarium of the Belgorod branch of the Federal Scientific Center for Experimental Veterinary Sciences, Russian Academy of Sciences, at a dose of 0.05 ml of suspension per 1 kg of body weight (1 ml of suspension contains 109 CFU). Biomaterial for biochemical and hematological analysis was collected on days 1, 5, 10, and 20 via intracardiac puncture, and body weight was measured during the same time interval. The main part of the morphological analysis was performed after creating an electronic image gallery using a Mirax Desk semiautomated microscope slide scanner (Carl Zeiss Microimaging GMbH, Germany), which allowed for maximum standardization of the morphometric study modes.It was found that enrofloxacin causes significant biochemical (increased bilirubin to 7– 10 μmol/L, decreased ALT by 34%) and morphological changes (increased liver weight by 21.7%), indicating hepatotoxicity. The introduction of a probiotic reduced the negative effects of the antibiotic: in the combined group, the liver weight was 13% lower, and ALT activity decreased by only 47.7%. The use of a probiotic alone improved metabolism (increased live weight by 18.4%, and reduced triglycerides) and accelerated hepatocyte differentiation. These findings support the use of B. subtilis to minimize the consequences of antibiotic therapy in poultry farming.