The antioxidant alpha -tocopherol (alpha -TOH) has been found to act as a pro-oxidant under many in vitro conditions. The observed tocopoherol-mediated peroxidation (TMP) is dependent on two primary factors. (1) Chain transfer: alpha -TO. radical reacts with lipid to form lipid peroxyl radicals. (2) Phase transfer: alpha -TOH can transport radical character into the lipoprotein. Given the limitations of existing initiators, there is a need for new compounds that avoid the requirement for alpha -TOH to act as a phase-transfer agent. We report here a study showing that the new unsymmetrical azo compound, C-8, initiates LDL lipid peroxidation without requirement for alpha -TOH. This initiator provides a steady source of free amphiphilic peroxyl radicals that efficiently initiates oxidation of alpha -TOH-depleted LDL at a rate comparable to that reported for the very reactive hydroxyl radical ((OH)-O-.). With other initiators tested, unsymmetrical C-12 and C-16 and symmetrical C-0 and MeOAMVN, alpha -TOH-depleted LDL displayed significant resistance to oxidation. Results indicate that the amphiphilic nature of the unsymmetrical initiators increases their partitioning into lipoprotein depending on the hydrocarbon chain length, and the symmetrical azo initiators C-0 and MeOAMVN primarily remain in the aqueous phase. Evidence suggests that even when the phase-transfer activity of alpha -TOH is limited, with the use of an initiator such as C-8, the mechanism of peroxidation remains controlled by TMP chain-transfer activity.