Pro-oxidative effect of phenolic antioxidant (vitamin E) in combination with the initiators on human low-density lipoprotein is known. Recently I reported that oxidative stress induced by vitamin E in combination with the herbicide paraquat enhances structural chromosomal damage in cultured anuran leukocytes. In the present study, the phenolic antioxidant vitamin E-synthetic-analogue 2,6-di-tert-butyl-p-cresol (BHT) in combination with paraquat was found to enhance structural chromosomal damage in cultured Pelophylax (Rana) nigromaculatus leukocytes more than paraquat only and paraquat plus nicotinamido adenine dinucleotido phosphate served as positive control, although BHT only had no effect on induction of structural chromosomal damage. Paraquat plus BHT-enhanced structural chromosomal damage was inhibited by combination of the superoxide dismutase mimic Mn(III)tetrakis(1-methyl-4-pyridyl)porphyrin and the hydrogen peroxide scavenger catalase. In test based on reduction of paraquat cation, BHT was found to reduce paraquat cation chemically to paraquat monocation radical. These results suggest that BHT functions in chemically donating electron to paraquat and thereby induces an acute accumulation of reactive oxygen species, resulting in increase in chromosomal damage.