The combined effect of radiation and YM-881 (SMANCS) was studied in vitro and in vivo. When 0.25 microgram/ml of YM-881 was simultaneously combined with radiation, during and after irradiation for 30 min in total, Dq decreased from 3.3 Gy to 1.4 Gy without changing D0 in the dose-survival curve of exponentially growing SCC VII tumor cells. Five or ten times administrations of 0.1 mg/kg YM-881 at an interval of 24 h did not inhibit tumor growth. However, administration of 0.1 mg/kg YM-881 just before every irradiation which was repeated five times at an interval of 24 h yielded dose modifying factors (DMFs) of 1.8-1.2 when the tumor response to treatment was evaluated by the time for the tumors to regrow to three times the original volume. Administration of YM-881 ten times just before every irradiation yielded DMFs of 1.3-1.2. Adverse effects of the combination on bone marrow were examined by spleen colony assay. After five injections of 0.1 mg/kg YM-881, the mean number of CFU-S per femur decreased to 77% of the pretreatment level, but this was not significant statistically (0.1 greater than p greater than 0.05). The slope of radiation response curve for CFU-S per femur was not affected by the combination.