Using Sal 57 S as test organism, the author studied biological effect of x-ray radiation by observing the lethal effect under various conditions, in which the state of the organism at irradiation, the properties of the material that gives secondary ray and the dosis of radiation were studied in different combinations. The lethal effect mentioned above was measured by enumeration of surviving cells and the following results were obtained. 1) The surviving curve obtained by the irradiation on cell suspension of the microorganisms could be represented in an exponential curve. In this study, several kinds of solutions were examined using as the liquid media of the bacterial suspension, i. e. normal saline, phosphate buffer, Fe++ added normal saline and Mg++ added normal saline. The survival rates obtained on each suspension were not different to a significant degree. However, the survival rate was somewhat decreased at the examination on the suspension into which platine wire was submerged. 2) The survival rates obtained by the x-ray irradiation on cell suspensions in different concentrations were quite the same on either suspensions. However, in comparative study between the irradiation on a homogeneous cell suspension, and that on unhomogeneous cell suspension, the later showed a higher survival rate. 3) The surviving curve obtained by the irradiation on a spreaded growth on the entire surface of nutrient agar was also repesentable by exponential curve. An application of lead plate as the material which gives secondary ray showed prominient reinforcement of lethal effect. The surviving curve vas deviated from the original curve to a downward concaved curve on semi-log co-ordinates. 4) The lethal effect to dry cells was inferior than that to wet cells and also the effect was reinforced by an application of lead plate as material which gives secondary ray. 5) The reinforcement of lethal effect by secondary ray was studied using several materials as the emitter for secondary ray. In this study the effect was determined by means of the growth observed on nutrient agar plate. As the result of this study, the effect of reinforcement came in following order Pb, Cu, Al then wooden plate; this order was the same as atomic numbers. The scondary ray could be easily intercepted even by a sheet of paper, acordingly, the lethal effect by the secondary ray was markedly decreased as an increase of the distance between the emitter and the surface of madia.