Studies were performed to determine the effect of red ginseng and white ginseng on jejunal crypt survival, endogenous spleen colony formation, and apoptosis of jejunal crypt cells in irradiated mice. The radioprotective effect of ginseng was compared with the effect of diethyldithocarbamate(D). Jejunal crypts were protected from irradiation by pretreatment of red ginseng (50 mg/kg B.W., I.P. at 36 and 12 hours before irradiation) and white ginseng (50 mg/kg B.W., I.P. at 36 and 12 hours before irradiation). Red ginseng administration before irradiation and both pretreatment and posttreatment (50 mg/kg B.W., I.P. at 30 minutes after irradiation) of white ginseng resulted in an increase of the formation of endogenous spleen colony. the frequency of radiation-induced apoptosis in intestinal crypt cells was also reduced by both pretreatment and posttreatment of red ginseng, and pretreatment of white ginseng. The radioprotective effect of DDC (1000 mg/kg B.W., I.P. at 30 minutes before irradiation) on jejunal crypt survival and apoptosis was similar to those of ginseng treatment. Treatment with DDC showed no significant modifying effects on formation of endogenous spleen colony. These results indicated that ginseng might be a useful radioprotector. Further studies are needed to characterize effective radioprotective components and mechanism of ginseng.