IL-1 and TNF have been found to act as protective agents from lethal doses of ionizing radiation in recent years. It was proposed that the mechanisms of the radioprotective effect of IL-1 and TNF may be resulted from to its selective stimulation of manganese supreoxide dismutase(Mn-SOD) expression on both RNA and protein level. Chinese hamster ovary(CHO) cells transfected with sense Mn-SOD cDNA showed decreased radiosensitivity after treatment with X-ray irradiation, whereas clones transfected with anti-sense Mn-SOD cDNA showed increased radiosensitivity. It was demonstrated that overexpression of Mn-SOD could promotes the survival of CHO cells from ionizing radiation.