The antibacterial activity of levofloxacin was compared with those of ofloxacin, ciprofloxacin, and other antibiotics. In general, levofloxacin was equally active or up to fourfold more active than ofloxacin against all 801 organisms tested. Levofloxacin was 64-fold more active than ciprofloxacin against Streptococcus pneumoniae and 2-to 4-fold more active than ciprofloxacin against Staphylococcus aureus, Xanthomonas maltophUia, and Bacteroides fragili&. Levofloxacin was two-to eightfold more active than ciprofloxacin against coagulasenegative staphylococci and Acinetobacter spp., although these improvements in potency may not be clinically relevant. Levofloxacin inhibited 90%o of streptococci when it was used at concentrations of 1 to 2 ;ag/ml. The therapeutic effects of levofloxacin were determined against acute systemic and localized infections in mice. For systemic infections, female mice (CF-1; weight, 20 2 g) * Corresponding author.were challenged with one 100% lethal dose (LD10o) by intraperitoneal injection of bacteria. Treatment was administered orally 1 and 3 h after infection. The 50% effective dose (ED50) was calculated on day 7 after infection. In another study, mice were challenged with 10Ox the LD50, and treatment was one dose given intravenously 1 h after infection.The efficacies of levofloxacin in pyelonephritis and pneumococcal lower respiratory tract infections (LRTIs) in mice were also determined and compared with those of ciprofloxacin. Pyelonephritis was established with Staphylococcus aureus by a previously described method (2). Treatment was oral, starting 1 and 4 h after infection, and was continued twice daily for a total of 4 days. Twenty-four hours after the last treatment, kidneys were excised, weighed, and homogenized, and viable bacterial counts were quantitated. LRTIs were established after nasal instillation of 2 x 107 CFU of Streptococcus pneumoniae into anesthetized mice. Oral treatment consisted of two doses given 24 h after infection. Twenty-four hours after treatment, the lungs were excised, weighed, and homogenized, and viable counts were quantitated. Concentrations of levofloxacin and ciprofloxacin in serum and tissue were assayed by an agar well diffusion method with E. coli OC 160 as the indicator organism.

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