In vitro activity of the quinolone grepafloxacin (OPC-17116) was compared with that of ciprofloxacin, fleroxacin, clindamycin, imipenem, and metronidazole by using the NCCLS-approved Brucella-base-laked blood agar dilution method and breakpoints, when available. Clindamycin, metronidazole, and imipenem inhibited > or = 98% of Bacteroides fragilis at the breakpoint; grepafloxacin, ciprofloxacin, and fleroxacin inhibited 83%, 6%, and 0, respectively, at 2 micrograms/ml. Grepafloxacin inhibited 39% of other B. fragilis group species isolated (80) at breakpoint (< or = 2 micrograms/ml) compared with 100% for metronidazole and imipenem, 83% for clindamycin, 6% for ciprofloxacin, and 1% for fleroxacin. Grepafloxacin demonstrated substantially better activity against B. fragilis than did ciprofloxacin or fleroxacin; overall activity against anaerobes was marginally better than that of ciprofloxacin or fleroxacin.