Human plasma haptoglobin (Hp) is classified according to three phenotypes: Hp 1-1, 2-1, and 2-2 attributed by their two common alleles 1 and 2. Clinically, the 2-2 phenotype is associated with the risk of cardiovascular diseases and diabetes mellitus in patients. In this study, we demonstrate that Hp is an extremely potent antioxidant, which directly protects low density lipoprotein from Cu(2+)-induced oxidation. Its potency was markedly superior to probucol (one of the most potent antioxidants). Ranking of the IC(50) of antioxidant activity was as follows: Hp 1-1 greater, similar Hp 2-1 greater, similar Hp 2-2 greater, similar probucol greater, similar vitamin E. Blockage of disulfide linkages between Hp subunits, not only abolished the alpha-helical content but also diminished the ability of Hp to form a complex with hemoglobin. The modified Hp subunits exerted almost 4 times greater antioxidant activity than that of native Hp. To investigate the antioxidant role of Hp on the cellular level, the cDNA of Hp 1-1 was cloned, introduced into the pcDNA3.0 vector which contains the cytomega lovirus promoter and transfected into chinese hamster ovary (CHO)-K1 cells. Following transfection, CHO cells were able to express Hp 1-1 protein and significantly (p < 0.001) elevated cell tolerance against oxidative stress. Transfected cells showed 2-fold higher resistance to hydrogen peroxide exposure for 24 h compared to control cells. Thus, Hp plays a provocative antioxidant role as demonstrated by our in vitro and ex vivo studies.