[en] Dry seeds of rice were exposed to thermal neutrons at the heavy water facility of Kyoto University Reactor (KUR), and to ⁶⁰Co gamma-rays in order to compare the radiobiological effect of thermal neutrons with that of gamma-rays. Seeds which had previously been immersed in an aqueous solution of 500 ppm boric acid and lithium chloride were also exposed to thermal neutrons. Chemical analysis was made for nitrogen, hydrogen and boron in rice seeds to calculate the absorbed dose in the tissue irradiated by thermal neutrons. Experimental results obtained on M₁-plant growth are as follows; 1) The value of thermal neutron flux equivalent to 1 rad of gamma-rays was approximately 1.4 x 10⁹Nsub(th)/cm², when calculated from the irradiation dose which would reduce seed fertility to 50 percent of that of the control. 2) The percentages captured radiation dose due to nitrogen, hydrogen and boron were estimated to be 41.9, 37.0 and 21.1 respectively in rice seeds. 3) The RBE value of thermal neutrons for seed fertility was estimated to be about 28. 4) Experimental data suggested that the RBE value itself was decreased with increased content of boron in tissues, although the degree of M₁ damage caused by thermal neutrons was increased about 4 times at its maximum by boron treatment. The effect of thermal neutrons was found to be smaller in lithium treatment than in boron treatment. (auth.)