Abstract We made a comparative study of the in-vitro activities of grepafloxacin (GPFX), ofloxacin (OFLX), erythromycin (EM), clarithromycin (CAM), roxithro-mycin (RXM), and minocycline (MINO) against 20 strains of Mycoplasma pneumoniae (17 clinical isolates and 3 standard strains). The minimum inhibitory concentration (MIC)90-to-minimum bactericidal concentration (MBC)90 ratio showed that the new quinolones have bactericidal effects on M. pneumoniae. Thus, it is expected that the new quinolones, especially grepafloxacin, will be clinically useful antimicrobial agents for the treatment of M. pneumoniae infection because of their good pharmacokinetic properties and bactericidal action.