The in vitro activities of cephalothin and cefoxitin against Bacteroides fragilis were studied by time-kill curves and measurement of residual drugs in culture supernatants. Cefoxitin was bactericidal, causing a decrease of 10(7) in viable counts over 24 hr. Cephalothin caused an initial decrease of 10(2) B. fragilis at 2 hr; this change was followed by growth of the organism within 24 hr back to the number present before addition of cephalothin. The concentration of cephalothin in broth decreased rapidly within 2 hr and was undetectable within 24 hr, whereas the level of cefoxitin decreased only 25% over the 24-hr period. Penetration of these drugs into perforated ping pong balls implanted intraperitoneally in rabbits was studied. Three weeks after implantation the reservoirs were infected with B. fragilis. After intramuscular administration of five doses of antibiotic, the penetration of cephalothin, as measured by bioassay, in uninfected and infected capsules was 16% and 2%, respectively, of the peak serum concentration; similar findings were noted with cefoxitin. For determination of the rate of breakdown within the infected site, radiolabeled antibiotic was injected into the capsule, and the concentrations of bioactive and radioactive drug were determined. With radiolabeled cephalothin there was a rapid decrease in bioactivity during the initial 60 min, and no active drug was measurable after 2 hr. In contrast, only 40% of cefoxitin was inactivated at the end of 6 hr. The results indicate that levels of cephalothin and cefoxitin are reduced significantly in sites infected with B. fragilis. The decrease appears to be mediated by both a decrease in penetration and inactivation at the site of infection.