The authors studied the bactericidal action and therapeutic effectiveness of ciprofloxacin in treating external ocular infections (bacterial conjunctivitis and bacterial blepharoconjunctivitis). 108 ambulatory patients with clinical signs of conjunctivitis and blepharoconjunctivitis were enrolled in the study. All subjects underwent a conjunctival swab before starting therapy and at 10 days, to identify the causative bacteria and their susceptibility to ciprofloxacin, following routine microbiological methods. The reported therapeutic success rate (95%) and bacteriological analysis confirmed the effectiveness of ciprofloxacin in subjects with bacterial conjunctivitis and bacterial blepharoconjunctivitis. In vitro tests conducted 10 days after treatment confirmed the in vivo therapeutic effectiveness, even for those infections characterized by a difficult etiological identification which interferes with specific antibiotic therapy.