The in-vitro activity of moxifloxacin, a new fluoroquinolone, against Chlamydia species was investigated. The minimal inhibitory concentration of moxifloxacin for 10 standard strains of different Chlamydia species and 15 wild-type strains of Chlamydia pneumoniae isolated in Japan, which were morphologically different from clinical isolates from the United States, ranged from 0.031 to 0.125 microg/ml. The activity of moxifloxacin was almost the same as those of sparfloxacin, and it was 16, 8, 2, and 2 times more active than ciprofloxacin, levofloxacin, grepafloxacin, and tosufloxacin, respectively. The minimal chlamydiacidal concentration of moxifloxacin ranged from 0.031 to 0.125 microg/ml. These results suggest that moxifloxacin has potential effects against Chlamydia species.