• Ciprofloxacin is a new quinolone antibiotic that is highly active in vitro against Pseudomonas aeruginosa. A rabbit model of bacterial keratitis was used to assess the in vivo efficacy of topical ciprofloxacin. Albino rabbits received intrastromal injections of 5 × 102 aminoglycoside-resistant P aeruginosa organisms. At five hours after inoculation, ciprofloxacin (3 mg/mL) therapy was initiated (one drop every 30 minutes for 12 hours). Corneal tissue was then excised for bacterial colony counts. No organisms were recovered from ciprofloxacin-treated eyes, compared with an average of 3.1 × 107 organisms per milliliter recovered from untreated controls. This model suggests that topical ciprofloxacin may be clinically useful in the treatment of aminoglycoside-resistant P aeruginosa keratitis.