The antibacterial activity of the new quinolone compounds enoxacin, norfloxacin, ofloxacin and ciprofloxacin was evaluated in 300 Enterobacteriaceae, 50 Pseu-domonasaeruginosa, 30 Acinetobacter spp., 15 Haemophilus influenzae, 50 Streptococcus faecalis, and 70 Staphylococcus aureus isolates and compared to that of nalidixic acid, gentamicin and various β-lactam compounds. Moreover, the rate of spontaneous mutants resistant to quinolone compounds was evaluated. In concentrations only insignificantly exceeding the minimal inhibitory concentrations (MIC), mutants could be isolated rather frequently (approx. 10-6 fold); in concentrations of at least 10 times the MIC resistant mutants were barely detectable. In general, the mutants exhibited a 4- to 8-fold increase of the MIC as compared to the wild strain. In S. faecalis mutants were not detectable, whereas they occurred in low frequency ( < 10-8 fold) in S. aureus strains. In all mutants there was almost, but not entirely, complete cross-resistance between the quinolone derivatives