Lactoperoxidase, in the presence of H2O2, I-, and rat liver microsomes, will peroxidize membrane lipids, as evidenced by malondialdehyde formation. Fe3+ assists in the formation of malondialdehyde. Fe3+ can be added at the end of the reaction period as well as at the beginning with equal effectiveness, suggesting that it only acts to assist in the conversion of lipid peroxides, previously formed by lactoperoxidase, to malondialdehyde. The addition of EDTA to the microsomal reaction mixture results in a 40% decrease in malondialdehyde formation. The antioxidant butylated hydroxytoluene will completely block the formation of malondialdehyde. Malondialdehyde formation is not dependent upon the production of superoxide, singlet oxygen, or hydroxyl radicals. Peroxidation of membrane lipids by this system is equally effective in both intact microsomes and in liposomes, indicating that iodination of microsomal protein is not required for lipid peroxidation to occur.