In this study the role of high density lipoproteins in lipoprotein peroxidation process was investigated. Under basal conditions, HDL isolated from human plasma or from total lipoprotein fraction (density > 1.21) using precipitation technique carried nearly 35-40% of the total plasma fatty acid peroxidation product (measured as malonaldehyde, MDA). HDL associated MDA was reduced to < 20% when HDL was isolated by ultracentrifugation from plasma treated with Cu++. Under these conditions, 45% of the cholesterol peroxidation products (oxysterols) were associated with HDL. HDL isolated from Cu++ treated plasma significantly lost its ability to inhibit LDL peroxidation. These results suggest that HDL plays an important role in lipid peroxidation a) by carrying significant amounts of cholesterol and lipid peroxidation products, and b) its ability to inhibit LDL oxidation is compromised when HDL itself is oxidized.