Ciprofloxacin is a new fluoroquinolone that is highly active against many diverse microorganisms. At concentrations of less than 1 microgram/mL it is active against most gram-negative bacteria, including Enterobacteriaceae, Haemophilus, Neisseria, and other Pasteurellaceae, Vibrionaceae, and various species of Pseudomonas and Acinetobacter. Most staphylococci, including strains resistant to methicillin, are also susceptible to ciprofloxacin. Streptococci are not highly susceptible to ciprofloxacin, and obligate anaerobes are generally resistant to this and other quinolones. Ciprofloxacin, like other quinolones, inhibits DNA gyrase, but its bactericidal effects are not completely reversible by inhibitors of protein or RNA synthesis. Thus, unlike many other quinolones, ciprofloxacin may have multiple lethal effects. Resistance is less readily selected in vitro by ciprofloxacin than by nalidixic acid, and single-step mutants usually remain susceptible to clinically achievable concentrations. Resistance mediated by mutations in genes altering DNA gyrase and expression of outer membrane proteins has been described for ciprofloxacin and other quinolones. The antimicrobial spectrum and potency of ciprofloxacin, coupled with its rapid bactericidal effects, make this fluoroquinolone a promising new antimicrobial agent.