Decrease of small intestinal damage caused by 5FU treated with a water-soluble extract from cultured medium ofGanoderma lucidum (Rei-shi) mycelia (MAK) andAgaricus blazeiMurill (AGA) was investigated in male B6C3F1. MAR and AGA at three different doses were mixed into basal diet into biscuits at 5, 2.5 and 1.25% and administered from 1 week before 250 or 500 mg kg of 5FU iv or ip injection. Three days after injection, animals were autopsied. Crypt survival of the small intestine in MAK groups was significantly increased as compared with 5FU values. Moreover, multinucleated giant cells in seminiferous tubes in 5% MAK value was significantly decreased as compared with 5% AGA value. These results suggest that MAK can act as a decrease of 5FU-induced damages.