Combination treatment of mice with misonidazole (0.5 mg/g body wt.) and hyperthermia (41.4 degrees C for 45 mins.) produced dramatic radiosensitization in hypoxic BP-8 murine sarcoma cells. The dose modifying factory (DMF: 4.3) was such that hypoxic BP-8 cells subjected to combination therapy became more radiosensitive than untreated, fully oxygenated cell populations. In contrast, radiosensitization by combination treatment was comparatively minor or completely absent in normal body tissues such as skin (DMF: 1.57), intestine (DMF: 1.0), and bone marrow (DMF: 1.0). These results suggest that simultaneous administration of misonidazole and hyperthermia may prove an effective adjuvant to conventional clinical radiation therapy.