The in-vitro antibacterial activities of pefloxacin, other 4-quinolones and representative β -lactams and aminoglycosides were assessed by determination of minimum inhibitory concentrations (MICs). Pefloxacin (MICs mostly 0.03–2 mg/1) was highly active against Enterobacteriaceae. Gentamicin had slightly lower activity, and ceftazidime and norfloxacin similar activities to pefloxacin whereas ciprofloxacin was more active. Pefloxacin (MICs 0.03–2 mg/1) was active against Acinetobacter but again ciprofloxacin was more active. Aeromonas was highly susceptible to pefloxacin and norfloxacin (MICs 0.008–003 mg/1) as well as to ciprofloxacin (MICs 0.001–0.008 mg/1). Pefloxacin (MICs 1–8 mg/1) had similar activities to ceftazidime and gentamicin against Pseudomonas aeruginosa but tobramycin (MICs 0.25–32 mg/1), norfloxacin (MICs 0.25–4 mg/1) and ciprofloxacin (MICs 0.06–1 mg/1) were generally more active. Haemophilus influenzae was susceptible to pefloxacin (MICs 0.008–0.06 mg/1) and to norfloxacin and ciprofloxacin, all of which were more active than ampicillin or ceftazidime. Gardnerella vaginalis was not very susceptible to pefloxacin (MICs 2–8 mg/1), the other 4-quinolones or gentamicin but ampicillin and ceftazidime were highly active. Neisseria gonorrhoeae was very susceptible to pefloxacin and norfloxacin (MICs 0.016–0.12 mg/1) and ciprofloxacin (MICs 0.002–0.008 mg/1). The activity of pefloxacin (MICs 0.25–1 mg/1) was similar to that of ciprofloxacin (MICs 0.12–2 mg/1) but greater than that of norfloxacin (MICs 0.5–4 mg/1) against Staphylococcus aureus . Vancomycin (MICs 1–2 mg/1) had similar activity in vitro but whilst gentamicin was highly active against some isolates, others were resistant. Pefloxacin (MICs mostly 4–32 mg/1) and the other 4-quinolones had lower activity against streptococci (including α-, β -, and non-haemolytic strains, enterococci and pneumococci) than against staphylococci. Benzylpenicillin (or ampicillin in the case of enterococci) were usually more active than any of the 4-quinolones. Bacteroides species, both of the fragilis and melaninogenicus/oralis groups were generally moderately resistant to pefloxacin (MICs 2–32 mg/1) and norfloxacin though ciprofloxacin was more active. Whilst the activity of pefloxacin and the other 4-quinolones was generally somewhat higher against the other anaerobes, ampicillin generally had greater activity.