Haptoglobin (Hp) is known as an acute phase protein in response to infection and inflammation.Two common alleles,Hp 1 and Hp 2,determine each individual with a phenotype Hp 1-1,2-1 or 2-2.We have shown that the protein is one of the most potent antioxidants that can directly inhibit Cu ((C)(o-))-or free radical-induced LDL oxidation in vitro.Transfection of HP cDNA into a Chinese Hamster Ovary (CHO) cells significantly elevates the tolerance of CHo cells against oxidative stress.Interestingly,remarkable levels of Hp are found to be accumulated in the human atherosclerotic lesions and-colocalized with LDL.For the first time,we demonstrate that LDL can stimulate Hp expression in smooth muscle cells and macrophages.