The activity of fleroxacin was compared to those of nalidixic acid, ampicillin, trimethoprim-sulphamethoxazole, tetracycline and penicillin against a total of 452 strains isolated from urinary tract and genital tract infections. These included strains resistant to gentamicin, methicillin-resistant Staphylococcus aureus and beta-lactamase producing strains. MICs were determined by a standard agar dilution procedure using Mueller-Hinton agar. Fleroxacin inhibited 90% of Escherichia coli, Klebsiella pneumoniae, Proteus spp., and Enterobacter spp. at less than or equal to 1 mg/l. All the gentamicin-resistant Enterobacteriaceae strains were highly susceptible to fleroxacin (MIC90 less than or equal to 0.5 mg/l). Against Pseudomonas, fleroxacin was the most active agent tested (MIC90 2 mg/l). All nalidixic acid-resistant strains were susceptible to fleroxacin (MIC90 less than or equal to 1 mg/l). The activity of fleroxacin was similar against methicillin-resistant and methicillin-susceptible strains of Staph. aureus (MIC90 less than or equal to 0.5 mg/l). Against Neisseria gonorrhoeae, fleroxacin (MIC90 0.12 mg/l) was as active as penicillin and tetracycline. However against N. gonorrhoeae strains producing beta-lactamase, fleroxacin (MIC90 0.5 mg/l) was at least twice as active as tetracycline. Fleroxacin is a potent quinolone with in-vitro activity against a variety of resistant bacterial species.