Bacteroides fragilis pure-culture abscesses and Bact. fragilis-Escherichia coli mixed-culture abscesses were initiated subcutaneously in mice and intraperitoneally in rats. Within 1 h after injection of Sch 34343, the drug was present in higher concentrations in the abscesses than in the blood of infected animals. After five days of Sch 34343 therapy with either 100 or 400 mg/kg administered five times a day to mice with subcutaneous abscesses, the numbers of Bact. fragilis in pus decreased approximately three log-fold, reflecting a killing of 99.99% of the viable Bact. fragilis, while the numbers of E. coli decreased approximately 0.5 log-fold, reflecting a killing of 50% of the viable E. coli. After five days of therapy with either 50 or 150 mg/kg administered five times a day to rats with intraperitoneal fibrin clot abscesses, the viable Bact. fragilis again decreased three log-fold; the viable E. coli decreased one log-fold in rats given the higher dosages of the drug. Sch 34343 is a promising agent for the treatment of anaerobic infections because it can penetrate into anaerobic abscesses and can kill large numbers of bacteria within abscesses.