The in vitro activity of garenoxacin (BMS-284756) against 62 clinical Haemophilus influenzae isolates with different fluoroquinolone susceptibilities was determined by the microdilution susceptibility testing method and compared with the activities of other oral quinolones and nonquinolone oral antimicrobial agents. Cefixime presented the highest intrinsic activity (MIC at which 50% of the isolates tested were inhibited [MIC(50)], 0.01 microg/ml), followed by garenoxacin, moxifloxacin, and ciprofloxacin (MIC(50), 0.06 microg/ml), levofloxacin (MIC(50), 0.12 microg/ml), cefuroxime (MIC(50), 1.0 microg/ml), and amoxicillin-clavulanate (MIC(50), 1.0/0.5 microg/ml), amoxicillin (MIC(50), 2 microg/ml), azithromycin (MIC(50), 4 microg/ml), and erythromycin (MIC(50), 8 microg/ml). In strains with ciprofloxacin MICs of < or =0.06 microg/ml, ciprofloxacin and garenoxacin displayed similar MIC(50)s and MIC(90)s, one dilution lower than those of moxifloxacin and levofloxacin. For strains for which ciprofloxacin MICs were > or = 0.12 microg/ml, MIC(50)s were similar for the four quinolones tested, although garenoxacin presented the widest activity range (0.03 to 32 microg/ml) and the highest MIC at which 90% of the isolates tested were inhibited (16.0 microg/ml). For strains without amino acid changes in the quinolone resistance determining region (QRDR) of GyrA and ParC, garenoxacin MICs were < or =0.03 microg/ml; with a single amino acid change in GyrA, garenoxacin MICs were 0.06 to 0.12 microg/ml; with one amino acid change each in GyrA and ParC, garenoxacin MICs were 0.5 to 2.0 micro g/ml; one amino acid change in ParC combined with two amino acid changes in GyrA increased the MICs to > or = 4 microg/ml for all assayed quinolones. We conclude that garenoxacin has excellent activity against H. influenzae, although progressive acquired resistance was observed by step-by-step mutation in the QRDR of gyrA and parC.