The in vitro activity of trovafloxacin against 238 clinical isolates obtained from respiratory specimens were compared with ciprofloxacin. Fifty-four Haemophilus influenzae, 58 Streptococcus pneumoniae, 40 Moraxella catarrhalis, 21 Streptococcus pyogenes, 25 Klebsiella pneumoniae and 40 Staphylococcus aureus (21 methicillin resistant and 19 methicillin susceptible) were studied. The MIC was determined following NCCLS recommendations. Ciprofloxacin and trovafloxacin MIC90 was 0.016 mg/l and 0.016 mg/l for H. influenzae, 4 mg/l and 0. 25 mg/l for S. pneumoniae, </=0.008 mg/l and </=0.008 mg/l for M. catarrhalis, 0.5 mg/l and 0.064 mg/l for S. pyogenes, 0.064 mg/l and 0.25 mg/l for K. pneumoniae, 16 mg/l and 0.5 mg/l for methicillin-resistant S. aureus and 2 mg/l and 0.032 mg/l for methicillin-susceptible S. aureus. According to this data, trovafloxacin showed better in vitro activity than ciprofloxacin in the Gram-positive pathogen studied and similar activity against the Gram-negatives tested.