Since most clinical radiotherapy is given as multiple small irradiation fractions, the present study was undertaken to test the in vivo radiosensitizing activity of a new hypoxic cell radiosensitizer, KU-2285, in combination with radiation dose fractionation. Radiosensitizing activity was measured by a growth delay assay using a transplanted mammary tumor in C3H/He mice, and by an in vivo-in vitro assay using the SCC VII tumor. KU-2285 was injected intraperitoneally 30 min before irradiation in all experiments. The in vivo-in vitro assay using SCC VII tumors showed that 12.5 micrograms/g of KU-2285 sensitized the tumors to irradiation (5 Gy/fr x 5 fr/48 hr or 6 Gy/fr x 3 fr/48 hr). KU-2285 also sensitized the transplanted mammary tumors to fractionated irradiation. We concluded that KU-2285 was able to sensitize two different murine tumors when given in combination with radiation dose fractionation.