A study was conducted to elucidate the possible role of singlet oxygen in pathogenesis of D-galactosamine-induced liver injury. Tissue and plasma levels of singlet oxygen were determined with chemiluminescence analysis. Following results were obtained: 1) Chemiluminescence as well as malondialdehyde, which is regarded as one of terminal products of lipid peroxidation, significantly increased in the liver and plasma of rats treated with D-galactosamine. 2) Elevation of plasma GPT and total bilirubin was also observed in rats with D-galactosamine-induced liver injury. Histological examination of the liver revealed submassive hepatic necrosis. 3) Administration of vitamin E, a radical scavenger of singlet oxygen, significantly inhibited the increases of chemiluminescence and MDA in the liver and plasma as well as the elevations of GPT and total bilirubin in the plasma. Histological changes of the liver were also found to improve significantly by vitamin E administration. In conclusion, singlet oxygen seems to be definitely involved, at least in part, in pathogenesis of liver damage induced by D-galactosamine. In addition, inhibition of the liver injury is possible, to some extent, by administration of vitamin E, one of the potent radical scavengers of singlet oxygen.