The in vitro activities of temafloxacin, ciprofloxacin, and ofloxacin against gram-negative bacteria are compared. The 90% minimal inhibitory concentrations (MIC90s) of temafloxacin for respiratory pathogens such as Haemophilus influenzae, Moraxella catarrhalis, Neisseria meningitidis, Bordetella pertussis, and Legionella pneumophila are less-than-or-equal-to 0.06-mu-g/mL. Temafloxacin is also active against bacterial agents of sexually transmitted diseases, including Neisseria gonorrhoeae (MIC90 less-than-or-equal-to 0.015-mu-g/mL) and Chlamydia trachomatis (MIC90 0.25-mu-g/mL). For strains of Enterobacteriaceae, Campylobacter, Vibrio, Aeromonas, and Acinetobacter, temafloxacin is generally inhibitory at less-than-or-equal-to 0.5-mu-g/mL. The MIC90 of temafloxacin for Pseudomonas aeruginosa is higher than that of ciprofloxacin, approximately 4-mu-g/mL versus 0.5-mu-g/mL. This activity, combined with its pharmacokinetic characteristics, should make temafloxacin an effective antimicrobial agent against infections caused by gram-negative bacteria.