Clinical isolates of respiratory tract pathogens were susceptibility tested against six different antimicrobial agents. The in vitro activity of moxifloxacin was compared with that of levofloxacin, cefaclor, amoxicillin-clavulanate acid, azithromycin and trimethoprim-sulfamethoxazole against 111 isolates, including Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis and other species isolated from respiratory tract infections. All isolates were susceptible to moxifloxacin, except for two isolates of Pseudomonas aeruginosa which showed intermediate-resistance (MIC=6µg/mL), and one isolate of Escherichia coli which showed resistance (MIC>32µg/mL). Only moxifloxacin and amoxicillin-clavulanic acid were active against 100% of S. pneumoniae isolates at the suceptible breakpoint (MIC90, 0.25 µg/mL and 0.064 µg/mL respectively). The rank order of the activity among this group of drugs against S. pneumoniae was as follows (% of susceptibility): moxifloxacin = amoxicillin-clavulanic acid (100%) > levofloxacin (97%) > cefaclor (71%) > trimethoprim-sulfamethoxazole (54%) > azithromycin (53%). Except for trimethoprim-sulfamethoxazole, all antimicrobial agents were 100% active against H. influenzae and M. catarrhalis. The fluoroquinolones, moxifloxacin and levofloxacin, were the most potent compounds against these pathogens (MIC(90) 0.032 0.19 µg/mL). These in vitro susceptibility testing data of moxifloxacin support the view that this fluoroquinolone will have an important therapeutic role in the treatment of respiratory tract diseases.