The dynamics of LPO marker malondialdehyde formation and peroxidase-destroying activity was studied in homogenized organs of guinea pigs, immunized with thermoextracts from S and L forms Brucella abortus I-206. The L form brucella thermoextract exhibited a lower reactogenicity and adequately activated the antioxidant system, due to which the destructive effects of ROS could be partially neutralized during the vaccinal process.