The in vitro activities of quinolones were determined with 36 strains of tetracycline- resistant, 39 strains of erythromycin-resistant, 21 strains of both resistant to tetracycline and erythromycin, and 27 strains of both susceptible to tetracycline and erythromycin of the M. pneumoniae isolates which have been isolated and classified in previous studies. The $MIC_{90}$ of moxifloxacin, levofloxacin, sparfloxacin, ofloxacin, ciprofloxacin, amikacin, and clarithromycin against 36 strains of tetracycline-resistant M. pneumoniae isolates were 0.125, 0.39, 0.125, 1.0, 1.0, 15.6, and $1.56\;{\mu}g/ml$, respectively. The $MIC_{90}$ against 39 strains of eryhtromycin-resistant M. pneumoniae isolates were 0.06, 0.39, 0.125, 1.0, 1.0, 15.6, and $25.0\;{\mu}g/ml$, respectively. The $MIC_{90}$ against 21 strains of both resistant to tetracycline and eryhtromycin of M. pneumoniae isolates were 0.125, 0.39, 0.125, 1.0, 1.0, 3.9, and $1.56\;{\mu}g/ml$, respectively. The $MIC_{90}$ against 27 strains of both susceptible to tetracycline and eryhtromycin of M. pneumoniae isolates were 0.125, 0.39, 0.125, 1.0, 1.0, 7.8, and $0.09\;{\mu}g/ml$, respectively. These result suggest that moxifloxacin, levofloxacin, and sparfloxacin of the quinolones might be promising antimicrobial agents for the treatment of tetracycline- and erythromycin-resistant M. pneumoniae infection. However, selection of ofloxacin, ciprofloxacin, and clarithromycin for the treatment of M. pneumoniae infection should be cautious decision with antimycoplasmal susceptibility test.