Antioxidative activities of vitamin E, ethylenediamine tetraacetic acid (EDTA), ferulic acid, and butylated hydroxy toluene (BHT) were monitored with lipid peroxidation systems. Formation of malonaldehyde (MA) and 4-hydroxy nonenal (4-HN) from ethyl linoleate or rat liver microsome oxidized by FeCl2/H2O2 or ADP/FeSO4 was measured by gas chromatography. Over 90% of MA and 4-HN formation was inhibited by 100 mmol/L of vitamin E. A synthetic antioxidant, BHT, showed the strongest antioxidative activity, followed by that of vitamin E, whereas EDTA accelerated formation of MA and 4-HN in the ethyl linoleate/FeCl2/H2O2 system. Vitamin E did not suppress lipid peroxidation significantly when microsome was oxidized by ADP/FeSO4. EDTA inhibited oxidation of microsome by ADP/FeSO4 considerably; in contrast, it did not in the case of oxidation by FeCl2/H2O2.