Objective To analyze the dose rate effect and potentially lethal damage repair in DNA double strand break repair deficient murine cells (SCID) irradiated by ? ray. Methods The wild type(CB.17+/+) and SCID cells were exposed to ? ray at high and low dose rates. The high dose rate exposure was fractionated into two equal doses at 24?h intervals. The survival rates of irradiated cells were calculated by clony forming analysis. Results When ? ray was given to wild type(CB.17+/+) cells in two fractions at 24?h intervals, the survival rate was significantly higher than that when the same total dose was given singly. In contrast, there was no difference in the survival rates between the single and fractionated exposure in SCID cells. SCID cells were more sensitive than CB.17+/+ cells to both low and high dose rates ? ray exposure for cell killing. The survival rate by low dose rate exposure was significantly higher than that by high dose rate exposure, not only in CB.17+/+ cells but also in SCID cells. Conclusions SCID cells are deficient in repairing ? ray induced double strand breaks. There is dose rate effect in both SCID and CB.17+/+ cells.