Immortalized cell lines from LEC strain (LEC) rats by SV40 large T antigen were more sensitive to X-irradiation than the cell lines from WKAH strain (WKAH) rats. The dose-response curves for inhibition of DNA synthesis in WKAH-derived cells showed a steep decline at low doses and a shallow decline at high doses. On the contrary, the dose-response curves for LEC-derived cell lines showed no steep component; they were almost identical to the shallow component of the curves for WKAH-derived cells.