A subcutaneous abscess model was utilized in investigating the usefulness of various antimicrobial combinations in the treatment of mixed aerobic-anaerobic infections in mice. The infected animals were treated for seven days with single or combined antimicrobial therapy. The single agents used were penicillin, clavulanic acid, oxacillin, metronidazole, and gentamicin. The antimicrobial combinations used were penicillin plus clavulanic acid and metronidazole plus penicillin, oxacillin, or gentamicin. Administration of a single agent was effective in treating abscesses caused by a single susceptible organism. The only effective therapies for mixed infections were the combinations of penicillin plus clavulanic acid or metronidazole plus gentamicin. This study demonstrated the efficacy of penicillin and clavulanic acid in the treatment of infections caused by Bacteroides fragilis alone or in combination with beta-lactamase-producing aerobic bacteria.