Weanling (Six weeks old) and adult (Six months old) white Smith rats were exposed to 500r whole body roentgenirradiation and the blood constituents were analysed for a period of 70 days. The reaction time and recovery ratesof the various blood constituents were compared for the weanling and adult rats. In addition to this examination,I attempted to observe whether the pure Ginseng powder possess the effect of protective action to x-rayirradiation.m (1) Weanling rats reponded more quickly and more drastically to whole body roentgen irradiation byleukopenia than did the adults, but they also recovered better after the irradiation insult. (2) LowestIymphopenia occurred by the first day in the weanlings and by the third day in the adults after irradiation. Moredrastical lymphopenia occurred in weanlings. Recovery is better in weanlings as in leukopenia. (3) Both in adultand weanling rats lost a high percentage of their neutrophiles by third days similary after irradiation, butweanlings recovered better than did adult at 49 days examination. This indicates more destructive changes ofneutrophiles of adult rats. (4) Erythrocytes were originally higher in adults than weanling rats. Erythropenia inits lowest level occurred in both weanlings and adults by the two weeks. The wealing showed better recovery andover compensation in 49days examination after irradiation. There was no evidence of better recovery of erythrocytein Ginseng abdoministered gruops compared to control groups. (5) Hemoglobin was originally higher in adults thanweanling rats and both responded to whole body roentgen irradiation with a maximum drop at 14 days. Hemoglobin inboth adults and weanlings showed overcompensation at 49 days examination after irradiation. There was no evidenceof better recovery of hemoglobin percentage in Giseng administered groups compared to control groups. (6)Platelets were originally higher in wealing rats than adult rats and responded to whole body roentgen irr