Exposure of isolated high-(HDL) and low-density lipoproteins (LDL) to aqueous peroxyl radicals generated from a thermo-labile azo-compound resulted in immediate formation of cholesteryllinoleate hydroxide (Ch18:2-OH) in addition to hydroperoxides of cholesteryllinoleate (Ch 18: 2- OOH) and phospholipids. Ch 18: 2- OH was also formed in peroxyl radical-oxidizing human plasma devoid of ascorbate or low molecular weight compounds or isolated lipoproteins in the presence of desferrioxamine. In contrast, peroxyl radical-mediated oxidation of HDL or LDL lipid extracts or detergent-solubilized lipoproteins resulted in the formation of Ch 18:2-OOH without concomitant formation of Ch 18:2-OH. Heat treatment of the isolated lipoproteins prior to oxidation greatly reduced Ch18:2-OH formation. Compared to the concentrations of Ch18:2-OOH accumulating, formation of Ch18:2-OH was more pronounced in oxidizing HDL than LDL isolated from the same blood donor. The levels of Ch18: 2-OH detected after prolonged oxidation periods were independent of the radical flux to which the lipoproteins were exposed. In the absence of peroxyl radical generator, [³H]Chl8: 2-OOH associated with HDL was converted readily and in a biphasic manner into [³H]Chl8: 2-OH upon incubation at 37 but not 4°C. LDL-associated [³H]Ch18: 2-OOH were also reduced, albeit with an initial reaction rate -10 times slower than that observed with labelled HDL. Together, the results show that cholesterylester hydroxides are formed during (peroxyl) radical-mediated oxidation of isolated intact HDL and LDL under transition metal-free conditions. The findings suggest the presence of a hydroperoxide reducing activity in isolated human lipoproteins, particularly HDL.