IUdR, a thymidine analogue, increased the radiosensitivity of SCC VII tumor cells. A decrease in Dq value of cell survival curve was more prominent compared with that in D0 value. However, in experimental liver tumors made by transplanting SCC VII tumors, the decrease in D0 value was apparent. Proportion of IUdR incorporated tumor cells in the liver increased with an increase in number of IUdR injections, and reached 52% after five administrations of the drug at an interval of twelve hours. However, few hepatocytes incorporated IUdR. The response of hepatocytes to radiation was evaluated by micronucleus appearance frequency. To accelerate the appearance of micronucleus in hepatocytes, partial hepatectomy was performed immediately after irradiations. No differences in micronucleus frequency were observed between the treated and untreated with IUdR.