[en] The effect of x-irradiation on the thymidine incorporation into the DNA of normal human blood lymphocytes was studied. It was found that the incorporation decreases with increasing x-ray dose. Assuming that the incorporation is proportional to the surviving cells, a simple exponential survival curve, consistent with the one-hit, one-target model was obtained. This survival curve had a D₃₇ of about 1030 rads. It was also found that the absolute response of the lymphocytes to phytohemagglutinin is reduced by x-irradiation by a constant factor, regardless of the phytohemagglutinin dose or the duration of the culture. It was concluded that the reduction of thymidine incorporation does not represent merely a delay of the initiation of the incorporation, but rather a decrease of the rate of incorporation. No difference in the effect of X-rays was observed when the cells were irradiated at different stages of their transformation