Activity of seven antimicrobial agents was examined using a mouse model of a subcutaneous infection that involved Bacteroides fragilis. Untreated mice had encapsulated abscesses with approximately 10(10) bacteria. Pharmacokinetic studies showed that all drugs tested penetrated into abscesses to provide mean peak levels that were 17% to 53% of mean peak serum levels. In vivo efficacy v 15 strains was measured by the reduction in counts of viable organisms when treatment was initiated one hour after challenge. This showed that the most active agents, in order of activity, were metronidazole hydrochloride, clindamycin phosphate, moxalactam disodium, and cefoxitin sodium. A delay in treatment of eight to 120 hours after challenge showed a noticeable reduction in activity, except with metronidazole. It is presumed that bacteria within an abscess are in a stationary growth phase, and this has an important influence on in vivo efficacy.