Radioprotective effects of a water-soluble extracts from cultured medium of Ganoderma lucidum (Rei-shi) mycelia (designed as MAK) and Agaricus blazei (Agaricus) against the shortening of survival time or the injury of crypt by X-irradiation were investigated in male B6C3F1 mice. MAK and Agaricus at three different doses were mixed into basal diet into biscuits at 5, 2.5 and 1.25% and administered from 1 week before irradiation. MAK (5% group) significantly prolonged animal survival as compared with basal diet group (control group) after 7 Gy of X-ray irradiation at a dose rate of 2 Gy min-1. At doses of 8, 10 and 12 Gy X-irradiation at a dose rate of 4 Gy min-1 MAK (5% group) significantly increased crypt survival as compared to other groups. These results suggest that MAK can act as a radioprotective agent.