Gas gangrene caused by Clostridium perfringens is associated with significant mortality and morbidity in spite of penicillin treatment. Although prompt surgical debridement has been established as the primary therapeutic objective, additional studies are needed for determination of the optimal antimicrobial therapy. In a mouse model of gas gangrene caused by Clostridium perfringens, clindamycin, metronidazole, rifampin, and tetracycline were all more efficacious than penicillin (P less than .05). Survival of penicillin-treated mice was not significantly better than that of untreated controls in spite of serum levels that ranged up to 77-1,800 micrograms/ml. Responses to metronidazole were highly dose dependent. For example, 60% of mice survived after 75 mg of metronidazole/kg, but only 10% survived after 19 mg/kg. In contrast, clindamycin was highly effective over a broad dosing range (8.6-86 mg/kg). The efficacy of all antibiotics was reduced if treatment was delayed or larger inocula of bacteria were used.