A highly effective test-system for quantitative characterization of the total antioxidant activity (TAA) of human blood serum (HBS), including methemalbumin (MetHa) as biocatalyst, H2O2 as the oxidant, o-phenylenediamine (PDA) as the acceptor of radicals and 2,2,5,7,8-pentamethylchroman-6-ol (PMC) as the inhibitor-calibrator, has been developed and proved under the laboratory environments. The test-system has been optimized for the concentrations of all the components, the reaction conditions and the PDA consumption monitoring at 37 degrees C in the medium of buffered physiological solution, pH 7.4 containing 5% DMFA and 0.5% DMSO. Under strongly standardized conditions by a comparison of the inhibiting action of the HBS and the calibrator PMC at 37 degrees C, the quantitative parameters of the HBS TAA were determined in microg of PMC, equivalent to 1 mg of HBS, or as a reverse value in mg of HBS, equivalent to the action of 1 microg of PMC. The values of the HBS TAA significantly varied for the group of healthy individuals and essentially decreased for the group of patients with thyroid gland pathologies. This underlies necessity of antioxidant therapy in these patients.