The in vitro and in vivo activities of AM-1155, a new quinolone, against Chlamydia spp. were investigated. The MIC of AM-1155 for 10 standard strains of different Chlamydia spp. and 25 wild-type strains of Chlamydia pneumoniae isolated in Japan, which were morphologically different from clinical isolates from the United States, ranged from 0.063 to 0.125 microg/ml. Its activity was almost the same as those of sparfloxacin and tosufloxacin and was 4 and 16 times superior to those of levofloxacin and ciprofloxacin, respectively, but lower than those of clarithromycin and minocycline (range for each, 0.016 to 0.031 microg/ml). The minimal chlamydiacidal concentration of AM-1155 ranged from 0.063 to 0.125 microg/ml, while those of clarithromycin and minocycline ranged from 0.016 to 0.031 microg/ml and 0.016 to 0.063 microg/ml, respectively. The therapeutic effect of a 7-day course of AM-1155 at doses of 5 and 10 mg/kg of body weight administered orally twice daily to mice with experimental Chlamydia psittaci pneumonia was excellent, with a 100% survival rate at 21 days after infection. The efficacy was equal to those of clarithromycin and minocycline and higher than those of ciprofloxacin and ofloxacin.