Chemiluminescent properties of blood serum from rabbits with hypo-, normo- and hyperthyroidism were investigated. The ferrum-induced chemiluminescence of the serum was shown to be decreased both in hypo- and in hyperthyroidism as compared with the normal state. The oxidative capacity of the main substrate of the peroxidation in blood serum--apo-B containing lipoproteins--did not markedly differ in various states of thyroid gland. The efficiency of branched free radical chains was dissimilar; these data suggest that the content of lipid-soluble antioxidants was increased in the lipoproteins in hypo- and decreased in hyperthyroidism. Measuring of the chemiluminescence quenching in the standard chemiluminescent system indicated that the content of water-soluble antioxidants was increased in blood of animals both with hypo- and hyperthyroidism. However, a higher content of water-soluble antioxidants was noted in blood of rabbits with hypothyroidism as compared with the animals with hyperthyroidism.