Studies were performed to determine whether the radiation sensitizer misonidazole (MISO) could enhance the tumor response of the KHT sarcoma to a treatment combining fractionated radiotherapy and the chemotherapeutic agent 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU). A single dose of CCNU (20 mg/kg) was given 24 hr prior to the start of a multi-fraction radiation protocol in which 10 fractions were delivered once a day in 12 days overall treatment time. Daily radiation doses ranged from 1.0 to 4.0 Gy. MISO was administered at a dose of 1.0 mmol/kg, either once as a chemopotentiator simultaneously with CCNU, or repeatedly as a radiosensitizer 30-40 min prior to each radiation dose. Tumor response to treatment was assessed using tumor regrowth delay as the end point. The results indicate that, at 1.0 mmol/kg, MISO failed to radiosensitize the tumors in each of the fractionation schedules evaluated; that is, there was no difference between the regrowth delay curves obtained when CCNU treatment was followed by fractionated radiation, administered either alone or with MISO prior to each radiation dose fraction. However, when a single dose of 1.0 mmol/kg MISO was combined with CCNU 24 hr prior to the start of radiotherapy, regrowth delay was increased for all fractionated radiation schedules, particularly, at the larger dose fraction sizes. Comparison of the dose response curves suggests that MISO, used as a chemopotentiator, effectively reduced the proportion of radiobiologically hypoxic cells in the tumors prior to the start of the radiation therapy. These findings indicate that chemopotentiation can be used effectively in a combination with fractionated radiotherapy.