A multiple-dose, randomized, double-blind, controlled, cross-over trial was performed in 12 healthy male subjects in order to investigate the effect of a 7-day treatment with moxifloxacin (400 mg orally, once daily) versus clarithromycin (500 mg orally, twice daily) on the normal oropharyngeal microflora. Moxifloxacin caused significant reductions in levels of alpha-streptococci and Neisseria cocci during the treatment period, while the numbers of gram-negative anaerobic bacteria increased markedly during moxifloxacin administration. Clarithromycin administration caused a suppression of micrococci and corynebacteria, while no significant changes were recorded in the anaerobic microflora. No new colonizing moxifloxacin-resistant strains were isolated during the investigation period.