A 65-kDa molecular mass of thiol-specific antioxidant protein was purified from human plasma and identified as human serum albumin (HSA) by the analysis of amino-terminal amino acid sequence. This protein exhibited the preventive effects against the inactivation of glutamine synthetase activity and the peroxidation of lipid by a metal-catalyzed oxidation system. These antioxidant activities were supported by a thiol-reducing equivalent such as DTT and reduced glutathione. The thiol-specific antioxidant activity of HSA was greatly activated by halide ion, especially by chloride ion. HSA showed a significant capability to destroy H2O2 in the presence of reduced glutathione, resulting in the production of oxidized glutathione. Both the preventive activity against the glutamine synthetase inactivation and the peroxidase activity were completely abolished by the reactions of HSA with N-ethylmaleimide and iodoacetate, chemical modification agents for sulfhydryl of protein, only in the presence of thiol-reducing equivalent such as DTT. These results suggest that serum albumin acts as a major and predominate antioxidant exerting a glutathione-linked thiol peroxidase activity which removes reactive oxygen species such as H2O2 within blood plasma.