For the radiosensitization of the repair rich epithelial cells of small intestine, BudR (B.U.) was continuously infused into mice. Its uptake into the DNA, as shown by the substitution rate (S.R.) of thymine to B.U., was dose-dependent in low dose, but above the 4 mg/d/head it did not increased. The radiosensitization effect was assayed by the clonogenecity of the cells, and (1) the repair capacity (recovery factor, RF.) of the cells and (2) the isoeffect dose (I.D) were obtained. From these data, their enhancement ratios (E.R) were estimated. E.R. by R.F. increased remarkably both in low dose and in low S.R., but showed the plateau at about 10 level after 4 mg/d/h of dose or 10% of S.R. Whereas E.R. by I.D. increased linearly in dose or it did quadratically in S.R. From these results, it was discussed that radiosensitization by debromozation of B.U. was seen in low dose, and the direct cytocidal effect of B.U. became dominant with the dose-increase. Also the clinical application of B.U. was discussed in cell kinetical aspect of B.U. labelling as well as the progress of infusion techniques.