The in-vitro activity of ciprofloxacin , a new 4-quinolone, was compared with that of norfloxacin and nalidixic acid against recent clinical isolates of medically important bacteria. Overall, ciprofloxacin and norfloxacin had similar spectra of activity, but ciprofloxacin had somewhat greater intrinsic activity against most organisms. Minimum inhibitory concentrations (MIC) were in the range 0.004-4 mg/l for Enterobacteriaceae, 0.016-2 mg/l for Pseudomonas spp., 0.004-1 mg/l for Acinetobacter, 0.004-0.016 mg/l for Haemophilus influenzae, 0.001-0.004 for gonococci, 0.004-0.03 mg/l for Cmpylobacter , 0.12-2 mg/l for staphylococci, 0.25-4 mg/l for streptococci, 1-32 mg/l for the Bacteroides fragilis group and mostly in the range 0.25-8 mg/l for other obligate anaerobes. Resistant variants could be selected readily in vitro by culture on any of the three compounds investigated. However, while the MICs of nalidixic acid were high for these variants those of ciprofloxacin and norfloxacin were much lower.