We have investigated the Cu2+ induced generation of lipid peroxides in low density lipoprotein (LDL) incubated with high density lipoprotein (HDL) and with purified paraoxonase, an enzyme normally resident on HDL. HDL (1.5 mg) and paraoxonase (20 micrograms) inhibited lipid peroxide generation in LDL by 32% and 25%, respectively after 24 h of incubation (both P < 0.01). The decrease in LDL lipid peroxides both with HDL and with paraoxonase were concentration dependent. The degree of protection offered by HDL tended to relate to its paraoxonase activity (R = 0.47; P < 0.06). Neither purified paraoxonase nor HDL chelated Cu2+ sufficiently to account for the decrease in LDL oxidation. Purified paraoxonase did not affect LDL oxidation when it had been heat inactivated. Mass transfer of lipid peroxides from LDL to HDL did not explain the protection of LDL against oxidation: the total lipid peroxides accumulating during incubation was decreased both by HDL and by paraoxonase. These results suggest a direct role for HDL in preventing atherosclerosis probably by an enzymic process which prevents the accumulation of lipid peroxides on LDL. Paraoxonase is an example of an enzyme which might possibly be involved.