Early administration of antibiotics may worsen the functioning of the turkeys’ antioxidant system. It was also assumed that the longer the time of administration of an antibiotic, e.g. a coccidiostat, the greater the risk of its accumulation in the liver. The study aimed to determine whether early administration of antibi- otics or feeding a diet containing coccidiostats causes accumulation in the liver and whether it affects the deterioration of the antioxidant system, and whether preventive vaccinations can intensify it. A total of 3 080 female turkeys were randomly allocated to eight groups. The experiment had a two-factorial design, with four treatments (C, M, E, D) and two groups of birds (vaccinated +, unvaccinated
). The C group did not receive the coccidiostat or antibiotics. Group M was administered monensin at 90 mg/ kg feed for 56 days of life. Group E received enrofloxacin at 10 mg/kg BW, and group D received doxycy- cline at 50 mg/kg BW, added to drinking water, for the first 5 days of life. One-day-old turkeys from groups C+, M+, E+, and D+ were administered live-attenuated vaccines against turkey rhinotracheitis and Newcastle disease by coarse spray; 28-day-old birds were administered a subcutaneously injected inactivated vaccine against Ornithobacterium rhinotracheale. Turkeys from groups C-, M-, E-, and D- were not vaccinated. It was determined that as a result of administration of enrofloxacin or doxycycline until the 5th day of life, biotransformation of these antibiotics occurred in the liver until the 56th day of life of the turkeys, which was confirmed by their lower level than the Maximum Residue Level. Because the con- centration of monensin in the liver of turkeys gradually increased with the extension of the time of its administration in the diet, it is probable that discontinuing its addition a day before the slaughter of birds will result in the presence of this coccidiostat in the liver of turkeys. Despite the accumulation of mon- ensin in the liver of turkeys, this coccidiostat did not increase oxidative reactions in the organism of tur- keys. Vaccination of turkeys can reduce oxidative reactions and apoptosis in the body. However, the effect of the redox system reaction is different immediately after vaccination, which is due to the mechanism of action of the immune system. If it is necessary to administer an antibiotic in the early rearing period, the effects of doxycycline on the organism’s immunity including antioxidant defence will be less severe than those of enrofloxacin.