[en] This report summarized the outcomes from a research project 'Effects of low-dose radiation' made by 5 groups. The effects of radiation on life prolonging were investigated in the atomic bomb victims and the mortality was significantly lower in the male victim group exposed to 50-99 cGy than the control. In addition, the relative risk of death due to any kind of causes was 0.93 for the males who were previously exposed to 10-19 cGy, lower than the non-exposed males. When pre-exposed mice to 5-10 cGy received radiation at LD₅₀ after 2 months, their survival rates were significantly higher than those pre-exposed to 2.5 cGy as well as the control mice, suggesting an increase in radio-resistivity by an appropriate low-dose pre-exposure. Further, it was observed that in Syrian hamster culture cells, the number of dicentric chromosomes produced by exposure at 1500 cGy were significantly reduced by pre-exposure to 2 cGy. Repeated radiation at a low dose (4 cGy) resulted to suppress the tumor growth in the mice grafted with MM₄₆ cells. These results suggest that a low dose radiation might enhance the immunological potency. Thus, it was confirmed that low-dose radiation has a hormesis effects as seen in chemical and physical factors. (M.N.)