The autoxidation of methyl linoleate in benzene at 37 degrees C by peroxyl radicals was found to generate hydroxyl radicals (center dot OH) from a secondary oxidation mechanism. The yield of hydroxyl radicals (similar to 2%) was determined by trapping these reactive radicals with benzene to give phenol. We propose that aC-H hydrogen abstraction from lipid hydroperoxides, the main autoxidation products, is the source of hydroxyl radicals.