Radiation-induced DNA double-stand breaks (DSBs) lead to numerous biological effects. To elucidate the molecular mechanisms involved in cellular responses to low dose and low dose-rate radiation, it is informative to clarify the roles of DSB repair related genes. In higher vertebrate cells, there are at least two major DSB repair pathways, namely non-homologous end-joining (NHEJ) and homologous recombination (HR). Here, it is shown that in chicken DT40 cells irradiated with gamma-rays at a low dose-rate (2.4 cGy/day), the growth delay in NHEJ-related KU70- and PRKDC (encoding DNA-PKcs)-defective cells were remarkably higher than in cells defective for the HR-related RAD51B and RAD54 genes. DNA-PKcs- defective human M059J cells also showed an obvious growth delay when compared to control M059K cells. RAD54(-/-)KU70(-/-) cells demonstrated their highest degree of growth delay after an X-irradiation with a high dose-rate of 0.9 Gy/min. However they showed a lower degree of growth delay than that seen in KU70(-/-) and PRKDC(-/-/-) cells exposed to low dose-rate irradiation. These findings indicate that cellular responses to low dose-rate radiation are remarkably different from those to high dose-rate radiation. The fact that both DT40 and mammalian NHEJ-defective cells were highly sensitive to low dose-rate radiation, provide a foundation for the concept that NHEJ-related factors may be useful as molecular markers to predict the sensitivity of humans to low dose-rate radiation.