Effect of single pre-administration of Cordyceps militaris(Cm) extract on the survival ratio, body weight and organ weight changes and blood cell counts after whole-body ${gamma}-irradiation$ were investigated. The single pre-administration of Cm extract at 24 hrs before ${gamma}-irradiation$ increased the 40-day survival ratio of irradiated mice from 60.1% to 71.4%. The administration of Cm extract completely prevented weight reductions of spleen and thymus produced by ${gamma}-irradiation$ (P<0.01, P<0.05). Similar but somewhat less radioprotective effect was also found In the testis of the Cm treated mice. The administration of Cm extract retarded the reduction of both leukocyte and lymphocyte counts occured during the first 7 days and accelerated the recovery of the counts thereafter. The exrtract also acclerated the recovery of the erythrocyte counts occured after the day 21th. SDS-polyacrylamide gel electrophoresis of the soluble proteins extracted from various organs did not reveal differences to any extent in all groups except in the livers of the irradiated and extract treated groups, in which some proteins were missing or less present. Also, the result of general intra and extra mycelial enzyme assays with Cm, extramycelial enzyme activity was relatively higher than the intramycelial enzyme, Cm appeared to indicate that ${alpha}-amylase$ was the highest among the enzymes and gluosidase and chitinase were followed. Since the spleen, thymus and testis have been well known as radiosensitive organs, the protective action of Cm extract on irradiated mice may be responsible for its enhancing recovery of these organs. Although the exact mechanism in protective effect of Cm extract on irradiated mice is not clear yet, the present study is the first report regarding the Cm which was tested and found to be a potential radioprotective agent.