The paper is to elucidate the effects of in vivo exposure to different levels of ,235,UO,2,F,2, on DNA repair capability and the mechanism. The DNA repair capacity, as measured by UV-induced unscheduled DNA synthesis (UDS) of splenic lymphocytes from inbred BALB/c male mice, was observed after injection of different doses of ,235,UO,2,F,2, into the caudal vein. UV-induced UDS of the splenic lymphocytes increased significantly (p,<,0.05 or p,<, 0.01) at doses of 0.1—20μg / kg body weight. Non-UV-induced UDS showed a significant increase (p,<,0.05 or p,<, 0.01) 12 days after ,235,UO,2,F,2, injection. The UV-induced UDS in PHA activated but non-proliferating (hydroxyurea treated) lymphocytes were lower than those of lymphocytes unexposed to PHA at the same doses of ,235,UO,2,F,2, (p,<,0.05 or p,<,0.01). Low doses of internally deposited ,235,UO,2,F,2, have a continuous DNA damage effect on the mouse lymphocytes, so that a significant stimulative effect on DNA repair capability in the cells is induced. The stimulative effect occurs only in a limited dose range, and the dose range of internally deposited ,235,UO,2,F,2, is larger than that of the external radiation. The DNA repair synthesis is significantly inhibited after UV-irradiation in PHA stimulated but non-proliferating lymphocytes.