The therapeutic efficacy of intravenous ciprofloxacin against experimentally induced systemic, respiratory tract and urinary tract infections was investigated in mice. The 50% effective dose (ED50) of intravenous ciprofloxacin against experimental systemic infections with Staphylococcus aureus Smith, Escherichia coli 444, Klebsiella pneumoniae KC-1, Serratia marcescens T-55 and Pseudomonas aeruginosa 15846 in mice, were 0.538, 0.0625, 0.0941, 0.294 and 7.76 mg/kg, respectively. These excellent results are equal to 6- to 17-fold the potency following oral administration. In murine respiratory tract infections with K. pneumoniae DT-S, the ED50 of intravenous ciprofloxacin was 20.1 mg/kg, and 26.2 mg/kg after oral administration. A marked decrease of viable bacteria count in lung was noted at doses above 7.4 mg/kg with intravenous administration. On the other hand, after oral dosing, although a marked decrease in viable bacteria count was observed with doses above 29 mg/kg, no decrease in counts was seen at 118 mg/kg and only a bacteriostatic effect could be noted. The therapeutic efficacy of intravenous ciprofloxacin against experimentally induced urinary tract infections in mice was compared to that after oral dosing by determining viable bacteria count in kidney 24h after inoculation. A dose-proportional decrease in counts was observed with both routes of administration at doses of 1.7, 5.6, 17 and 56 mg/kg. Based on these results, intravenous ciprofloxacin was found to have effects superior to those after oral administration in all infection models studied.