The pharmacokinetic properties of 10 antimicrobial agents were examined in sterile and infected encapsulated subcutaneous abscesses in mice. The inoculum for sterile abscesses was autoclaved cecal contents; that for infected abscesses was autoclaved cecal contents combined with Bacteroides fragilis. The antimicrobial agents examined were rosaramicin, clindamycin, chloramphenicol, metronidazole, and six β-lactam antibiotics. All antimicrobial agents entered abscesses, produced peak levels of biological activity that were somewhat delayed in comparison to serum levels, and were present in negligible levels 8 hr after administration. The highest concentration in abscesses was achieved with rosaramicin and clindamycin, with peak levels of 43%–63% of the peak serum level. Peak levels of other antimicrobial agents in sterile abscesses were 13%–27% of the peak serum level. Levels of biologically active drug were significantly lower in infected abscesses than in sterile abscesses for antimicrobial agents that are inactivated by B. fragilis β-lactamase.