It has been shown that in the solution of low density lipoproteins (LDL) during their incubation at 37 degrees C the turbidity and concentration of malondialdehyde was increased, as compared to that observed at 4 degrees C. Both parameters were slowed down by the addition of high density lipoproteins (HDL) into the medium. The protective effect of HDL depended on the time of incubation and the concentration of HDL added. Delipidated HDL had no effect. Similar action of HDL was established in the experiments where the peroxidation in LDL was induced by the xanthine-xanthine oxidase. The data obtained demonstrate that HDL possess an antioxidant property that may play an important role in their antiatherogenic action.