The radiation induced bystander response is defined as a response in cells which have not been directly targeted by radiation, but which are in the neighborhood of cells which have been directly exposed. In many cases, the bystander response is saturated with increasing dose and is observed when only one cell in a population is targeted by high-LET particle radiations or ultrasoft X-rays (278 eV). However, in our studies using synchrotron X-ray microbeams (5.35 keV), the bystander cell killing effect in normal human fibroblast WI-38 cells had a parabolic relationship to the irradiating dose and was detected if 5 or more cell nuclei were irradiated. To evaluate the feature of the X-ray-induced bystander cell killing effect at a wider dose range and the existence of photon energy dependence, the effects were assessed by irradiating cell nuclei in confluent WI-38 cells with AlK X-ray microbeams (1.49 keV). The surviving fraction decreased when only a single cell nucleus was irradiated, suggesting the minimal number of targeted cells to induce the effect may depend on the energy of photons used. In this study, we found that the bystander cell killing effect showed a biphasic relationship to the irradiating dose. The decrease in bystander cell survival at the doses higher than 0.23 Gy was partially suppressed between 2.3 and 7.0 Gy, followed by level-off around 90% above 14 Gy, suggesting that the X-ray-induced bystander response is dose dependent. In addition, NO is one of chief initiators/mediators of the effect at least 0.47 Gy.