The peroxidation of low density lipoprotein (LDL) may play an important role in the modification of the lipoprotein to an atherogenic form. The oxidation of LDL by peroxidases has recently been suggested as a model for in vivo transition metal ion-independent oxidation of LDL (Wieland, E., S. Pathasarathy, and D. Steinberg, 1993, Proc. Natl. Acad. Sci. USA, 90: 5929- 5933). It is possible that in vivo the peroxidase activities of proteins, such as prostaglandin synthase and myeloperoxidase, promote LDL oxidation. We have used horseradish peroxidase (HRP) and H2O2 as a model of peroxidase- dependent oxidation of LDL and we observed the following during HRP/H2O2- initiated LDL oxidation. i) The oxidation of α-tocopherol occurred with the concomitant formation of α-tocopheroxyl radical. This was followed by the production of an apolipoprotein B (apoB)-derived radical. The apoB radical and the α-tocopheroxyl radical were formed under both aerobic and anaerobic conditions. ii) Inclusion of N-t-butyl-α-phenylnitrone (PBN) did not inhibit α-tocopheroxyl radical formation. The ESR spectrum of a PBN/LDL-lipid derived adduct was observed after prolonged incubation. iii) There was formation of conjugated dienes, lipid hydroperoxides and thiobarbituric acid reactive substance. Our data indicate that HRP/H2O2 oxidizes both α- tocopherol and apoB to the corresponding radicals and concomitantly initiates lipid peroxidation.