HDL protects LDL from oxidative damage known to contribute to the onset and progression of atherosclerosis. This antioxidant protection by HDL is mediated by intrinsic hydrolytic enzyme systems, by accelerated selective uptake of lipid peroxides from HDL and by antioxidant molecules. Oxidative damage suffered by HDL as a result of the transfer of reactive species from LDL may compromise HDL function in cholesterol transport.