The protective effect of selenium against irradiation were studied by measuring the content of lipid peroxide (LPO) and activity of glutathione peroxidase (GSH-Px) in liver and spleen of BALB/C mice. The results showed that LPO contents in liver and spleen of whole-body-irradiated mice were higher than those of the control group on the third day after irradiation with 3 Gy 60Co ?-ray, while the GSH-Px activity of these tissues was significantly decreased. However, when the animals were given sodium selenite solution 0.2ml (12.5?g selenium) per day via mouth successively for 7 days before irradiation, there were remarkably reduced LPO level and increased GSH-Px activity in liver and spleen of irradiated mice. It is suggested that selenium supplementation may prevent mice from radiation injury by reducing LPO level and increasing GSH-Px activity in liver and spleen.