Cell survival assays were performed to evaluate the effects of radiations released during neutron capture reactions by gadolinium-157, boron-10 and by the combination of both.Single cell suspensions with or without Gd-157 and/or B-10 were exposed to thermal neutrons produced by the Kyoto University reactor, and standard cell survival curves were obtained.Under the same molarity, cytocidal effects were 1.5 times greater for Gd-157 than for boron when compared at 10% survival levels. The presence of B-10 enhanced the radiation effect of Gd-157 neutron capture by 1.2-fold, suggesting that cells were not sufficiently irradiated as a result of neutron fluency attenuation by the presence of excess neutron capture agents in the medium.When an equal number of atoms were present, Gd-157 was effective as B-10 when exposed to an equal number of thermal neutrons. However, there was no benefit observed in the combination of Gd-157 and B-10 for neutron capture therapy. Further studies are needed to determine optimal Gd-157 and B-10 concentrations as a function of tumor dimension.