Aspiration pneumonitis and lung abscess generally involve anaerobic bacteria, which normally colonize the upper respiratory passages. The therapeutic response of these infections to parenteral penicillin G (49 patients) and parenteral clindamycin (35 patients) was compared to determine relative efficacy. No difference was discerned between these two agents in terms of time required for defervescence, roentgenographic clearing, and ultimate outcome. Seven patients with infections including Bacteroides fragilis were treated with penicillin G, and all responded well. These data indicate that penicillin G is the preferred agent for pulmonary infections involving anaerobic bacteria. Clindamycin is a suitable alternative for patients in whom penicillin G is contraindicated.