The in-vitro activity of norfioxacin was compared to that of nalidixic acid, cinoxacin trimethoprim, trimethoprim/sulphamethoxazole, ampicillin, cephalexin, doxycycline, amikacin, and gentamicin against 205 isolates of Gram-negative and Gram-positive bacteria. Norfloxacin was the most effective agent against all of the organisms tested except enterococci and Staphylococcus aureus. The minimal inhibitory concentrations at which at least 90% of the organisms were inhibited (MIC90 were 0·5 mg/l for all Enterobacteriaceae and Pseudomonas aeruginosa, 1·0 mg/l for all staphylococci, and 4·0 mg/l for enterococci. In addition, the activity of norfloxacin was compared to that of penicillin G, ampicillin, cefonicid, cefoxitin, cefuroxime, doxycycline, and spectinomycin against 104 clinical isolates of Neisseria gonorrhoeae. Norfloxacin was the most active with an MIC90 of 0·03 mg/l.