Background: Respiratory pathogens are becoming increasingly resistant to antimicrobials. A new group of drugs, called respiratory quinolones have been synthesized to overcome this problem. Aim: To study the in vitro susceptibility of respiratory pathogens to old and new antimicrobials. Material and methods: Forty five strains of S pneumoniae, 44 strains of H influenzae, 21 strains of M catarrhalis, 10 strains of methicillin susceptible S aureus and 20 strains of methicillin resistant S aureus were studied. All were isolated from community acquired respiratory infections during 1999. Minimal inhibitory concentrations of moxifloxacin, amoxicilin, amoxicilin/clavulanic acid, clarithromycin, azithromycin, ciprofloxacin and levofloxacin were determined using the Etest method. Betalactamase production by H influenzae and M catarrhalis was also studied. Results: S pneumoniae strains were 100% susceptible to quinolones and cotrimoxazole, 2% were resistant to macrolides, 11% were resistant to amoxicilin/clavulanic acid and 47% were resistant to cefuroxime. H influenzae was 100% susceptible to quinolones, azithromycin and amoxicilin/clavulanic acid. There was a 53% resistance to cotrimoxazole, 21% to amoxicilin, 9% to clarithromycin and 7% to cefuroxime. M catarrhalis was 100% susceptible to quinolones and 100% resistant to amoxicilin, 5% resistant to macrolides, 14% resistant to amoxicilin/clavulanic acid, 20% to cefuroxime and 30% to cotrimoxazole. Methicilline susceptible S aureus was susceptible to all antimicrobials and methicillin resistant S aureus was resistant to all. Conclusions: Moxifloxacin and the new respiratory quinolones can be useful in the treatment of respiratory infections (Rev Méd Chile 2000; 128: 1319-26).