Carbenicillin, chloramphenicol, doxycycline, and clindamycin were compared with penicillin for the treatment of lung abscess in an animal model produced by transtracheal inoculation of a mixture of anaerobes: Bacteroides fragilis, Peptococcus morbillorum, Eubacterium lentum and Fusobacterium nucleatum. Both chloramphenicol and doxycycline eliminated the bacteria but failed to close the abscess cavity. Carbenicillin, although it eradicated B. fragilis, failed to close the abscess cavity in 3 of 6 animals. In all animals tested, clindamycin sterilized the abscess cavities and healed the lung abscesses, while penicillin failed to eradicate the infection. Clindamycin was significantly more effective than penicillin in the elimination of anaerobic bacteria from the lung (p less than 0.05). Clindamycin also closed the abscess cavity faster than penicillin (p less than or equal to 0.02). The superior efficacy of clindamycin may have been the result of accumulation in the lung tissue in concentrations four- to eightfold higher than in the serum.