In this paper the effects of exogenous SOD on differentiation, proliferation, regulation of subpopulation of spleen lymphocytes has been studied with irradiated tumor−bearing mice. The activity of antioxidant enzymes and the contents of redoxic products have also been determined. And the effect of SOD on ultra−structure of spleen of irradiated tumor−bearing mice has been investigated meanwhile the killed effect of spleen tumor under irradiation with addition of SOD has been observed. It has been found that SOD does emphasize the differentiation, proliferation, regulation of subpopulation of spleen lymphocytes cell with radiated tumor−bearing mice and increase the activities of GSH、GPX and total−vigor of SOD while MDA decreased in spleen. It has been also found that SOD injected before or after irradiation can reduce the injury of spleen obviously and makes the tissue have higher protection from irradiation while the tumor was killed as the same as no addition of SOD. It can be deduced that lighten of lipid peroxidation occurs from the scavenge of ROS by SOD, the high protection of sleep from irradiation is because of delay to S phase of lymphocytes and increase of activity of antioxidant enzymes. The concentration change of SOD and ROS may have some influence on signal transferring pathway and gene expression of cell apoptosis.