The efficacies of five common antimicrobial agents were determined for a pure Bacteroides fragilis infection in mice. Therapy was begun 4 h after bacterial injection and given every 8 h thereafter for 5 days. Blood levels were determined over an 8-h period for each concentration of antibiotic tested. Clindamycin and tetracycline were the most effective in preventing the formation of abscesses. Chloramphenicol, penicillin G, and cephalothin were not effective in protecting the mice from infection.