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Background: The in vitro antimicrobial activities of new fluoroquinolones were tested against quinolone-resistant Haemophilus influenzae of clinical isolates. Methods: The nucleotide sequences of the gyrA and parC genes from three ciprofloxacin-resistant strains of Haemophilus influenzae (MIC, 1.56-6.25µg/ml) were determined. The gyrase was purified from the clinical isolates, and the inhibitory activities of quinolones against the enzyme were tested. Results: These strains possessed at least one amino acid substitution in each of the GyrA (asparagine at residue 88 (Asp-88) to Tyr, Ser-84 to Leu or Ser-84 to Leu and Asp-88 to Asn) and ParC (Glu-88 to Lys). The antibacterial activity of olamufloxacin against the resistant strains was most potent compared with other quinolones, and the inhibitory activities correlated with quinolone resistance of these strains. Conclusions: These results warrant the clinical effects of new types of fluoroquinolones, such as olamufloxacin, against respiratory tract and otolaryngology infections caused by ciprofloxacin-resistant H. influenzae. [ABSTRACT FROM AUTHOR]