The activity of the fluoroquinolone antibiotics against anaerobic bacteria has generally been moderate to poor. Nalidixic acid and cinoxacin show poor activity against almost all anaerobes. Norfloxacin and enoxacin exhibit poor anaerobic activity (MIC90, 8- > 256 micrograms/mL) except against Clostridium perfringens, Bacteroides ureolyticus, and some eubacteria. Norfloxacin is slightly more active than enoxacin against some Bacteroides species. While ciprofloxacin is more active than earlier fluoroquinolones against anaerobes such as Bacteroides fragilis (MIC90, 4-16 micrograms/mL), fusobacteria, and peptostreptococci, its activity is often variable and its MIC90 is frequently close to the maximal level attainable in serum. Ofloxacin is active against B. fragilis (MIC90, 4-8 micrograms/mL) but not against other species of the B. fragilis group (MIC90, 8-32 micrograms/mL); other anaerobes (e.g., peptostreptococci and fusobacteria) are generally susceptible to < or = 8 micrograms of ofloxacin/mL. Several newer quinolones exhibit improved anaerobic activity (although the studies yielding relevant data have often used diverse methods, small numbers and varieties of isolates, and different breakpoints). CI-960 and WIN 57273 inhibit 95% of strains tested at < or = 2 micrograms/mL. CI-990, sparfloxacin, and temafloxacin inhibit most anaerobes at < or = 2 micrograms/mL, but clustering around the breakpoint and strain variability have been noted. CI-990 is active against more than 85% of B. fragilis strains (MIC90, 4 micrograms/mL), but other species of the B. fragilis group often require > 4 micrograms/mL for inhibition; Prevotella bivia, Fusobacterium varium, and Fusobacterium ulcerans are usually resistant.(ABSTRACT TRUNCATED AT 250 WORDS)