The host-defense mechanisms against cancers are known to be modulated by 
changing the environmental factor(s). The spontaneous incidence of myeloid 
leukemia is about 1% in C3H/He mice, and the incidence increases up to 23.3% 
when a single dose of radiation, 3 Gy X-ray, is exposed to a whole-body. Since 
calorie restriction was known to reduce the incidence of spontaneous tumors, a 
question as to whether such radiation induced-increase of myeloid leukemia would 
be also decreased by calorie restriction, was aimed to answer to elucidate 
possible mechanism of radiation-induced myeloid leukemia. By the calorie 
restriction, the incidence of myeloid leukemia was significantly decreased. In 
addition, the latent period of the myeloid leukemia in the groups for calorie 
restriction was significantly extended at a greater extent as compared with the 
control diet groups. Number of hemopoietic stem cells, the possible target cells 
for radiation-induced leukemias, in the groups for the calorie restriction 
demonstrated a significant decrease, especially in the spleen, as compared with 
that in the control, when the evaluation was made at the time of radiation 
exposure.