The bacteriostatic or bactericidal action of metronidazole, clindamycin, tetracycline, chloramphenicol, cefoxitin and penicillin were determined against sixteen Bacteroides fragilis strains and six B. thetaiotaomicron strains. Drugs were considered bactericidal when minimal bactericidal concentration (MBC) to minimal inhibitory concentration (MIC) ratios were less than or equal to four. As expected, metronidazole, cefoxitin and penicillin were bactericidal. However, clindamycin and chloramphenicol, which other investigators have considered as bacteriostatic antibiotics, were bactericidal. Tetracycline was irregularly bactericidal. When tested against Escherichia coli, tetracycline and chloramphenicol were bacteriostatic as reported by others. The MBCs of metronidazole, clindamycin and tetracycline were well within the clinically achievable levels of each drug. Death rate data confirmed these findings. With tetracycline, chloramphenicol and metronidazole MICs and MBCs increased as inocula sizes increased, however, with clindamycin and cefoxitin only MBCs increased. Clindamycin was administered to mice which had pure B. fragilis abscesses. Over 8 h cell numbers in the pus remained relatively constant even though, during this same period, the concentration of clindamycin in the pus was above the MBC of this drug for the strain used. / M.S.