The best means to prevent post-transfusion GVHD is prophylactic irradiation of blood. However, the activation to the irradiated blood is not yet studied. We analyzed the radioactivity of blood and blood bag after irradiation to X-rays and electron beams. It is found that a blood-bag is radioactivated by 10MV X-rays. However, 4MV X-rays and electrons below 15MeV did not activate blood-bag. From ... [Show full abstract] the analysis by gamma-ray spectrometer and β ray measurement, we have concluded that neutrons contaminated in 10MV X- rays react with 37Cl containing in a blood-bag and produce 38Cl from 37Cl (n, γ) reaction. Therefore, to avoid radioactivation, the blood irradiation by gamma-rays, X-rays below 6MV, electron beams are recommended.