Hematopoietic residual injury, which is identified by the permanent defect of the blood forming system, could be induced in mice by repeated sublethal ,60,Co γ-ray exposures. The number of hematopoietic stem cells (CFU-S) in mouse femora could be enhanced to a normal or near normal level if the 3 or 5 doses of 4.0 Gy irradiated mice were treated with a preparation extracted from an edible fungus Tremella Fuciformis Berk (TFP) 2 months after the irradiation. The number of endogenous spleen colonies and the spleen weight also increased markedly after the treatment. Because of the similarity found in the changes of hematopoietic functions between the mice of residual injury and the patients of aplastic anemia, it is suggested that TFP may be useful to the patients of such blood disorder.