After UV-irradiation, no difference in the survival curves was observed among cell lines derived from LEC strain (LEC) rats and WKAH strain (WKAH) rats. The dose-response curves for inhibition of DNA synthesis in WKAH-derived cells showed a sharp decline at lower doses and a mild decline at higher doses of UV-rays. In contrast, the dose-response curves in LEC-derived cell lines had no sharp component, and were almost identical to the mild component of the curves in WKAH-derived cells. These results show that DNA synthesis in the cell lines of LEC rats was more resistant to UV-irradiation than that of WKAH rats.