The effect of twenty amino acids on the radical intensity of four antioxidants (sodium L-ascorbate, sodium 5,6-benzylidene-L-ascorbate, gallic acid, caffeic acid) was investigated, using ESR spectroscopy. Methionine and methional did not significantly affect the radical intensity of these antioxidants. Methionine sulfoxide slightly enhanced the radical intensity of sodium ascorbate and sodium 5,6-benzylidene-L-ascorbate, but did not that of gallic acid and caffeic acid. Cysteine, N-acetyl cysteine and glutathione significantly reduced the radical intensity and cytotoxic activity of these antioxidants except for sodium 5,6-benzylidene-L-ascorbate. The other amino acids were inactive. The present study further supports that these antioxidants induce cytotoxicity via their pro-oxidant action.