The in vivo activity of olamufloxacin (HSR-903), a new fluoroquinolone, was evaluated and compared with ciprofloxacin, sparfloxacin and levofloxacin. Olamufloxacin was active against systemic infection in mice inoculated with both Gram-positive and -negative bacteria. Olamufloxacin had equal efficacy for experimental urinary tract infections in mice caused by Pseudomonas aeruginosa.