The in vitro activities of ofloxacin alone and in combination with metronidazole against 177 anaerobic bacteria isolated from intra-abdominal infections, as determined by broth microdilution, showed that some Bacteroides fragilis strains were susceptible and that most other B. fragilis group species strains were resistant to ofloxacin. Isolates of other anaerobic species and genera, including those causing female genital tract disease, were generally susceptible to ofloxacin. Ofloxacin in combination with metronidazole usually showed an additive or indifferent interaction but no antagonism.