The effect of anti-cancer treatment on the AH136B tumor was studied using external beam irradiation in combination with a new oil-soluble agent, 3', 5'-dioctanoyl-5-bromodeoxyuridine (BrdU-C8), a lipophilic prodrug of BrdU. BrdU-C8 was dissolved in an oily lymphographic agent, Lipiodol (BrdU-C8/Lipiodol). BrdU-C8/Lipiodol is selectively accumulated in the neovasculature of the tumor and gradually releases BrdU. The AH136B tumor cell, a transplantable rat ascites hepatoma cell line, was implanted in the dorsal foot of rats. In vivo labeling index (L.I.) of the tumor cells after the intraarterial infusion of BrdU-C8/Lipiodol was significantly increased compared to the L.I. observed after intraarterial or intravenous infusion of BrdU. In addition, X-ray irradiation in combination with intraarterial infusion of BrdU-C8/Lipiodol significantly inhibited the tumor growth, indicating the increased radiosensitizing effect.