Local control of a mammary carcinoma in air-breathing C3H mice was enhanced by giving metronidazole before radiotherapy; When 0.5-1.0 mg metronidazole/g body weight was given ip 10 minutes before irradiation, the dose required for local control of 50% of the tumors (TCD50) was reduced by a factor of 1.37-1.38. The TCD50 was reduced by a factor of 1.48-1.50 when tumors were irradiated 30-60 minutes after 1.0 mg/g. Metronidazole alone, at a dose of 1.0 mg/g, only slightly inhibited tumor growth. The acute skin response of the leg, as determined by the radiation dose required to produce complete moist desquamation in 50% of the animals (DD50), was enhanced by a factor of 1.26 when mice were irradiated 10 minutes after a dose of 1.0 mg metronidazole/g. Clonogenic cells in the jejunal epithelium were not sensitized after metronidazole administration in either air-breathing mice or those irradiated in O2 30 pounds per square inch, but the drug appeared to exert a slight protective effect at higher doses of radiation.