Some patients with Mycoplasma pneumoniae infection are clinically resistant to antibiotics such as erythromycin, clarithromycin, or clindamycin. We isolated M. pneumoniae from such patients and found that one of three isolates showed a point mutation in the 23S rRNA gene. Furthermore, 141 EM-sensitive clinical isolates of M. pneumoniae were cultured in broth medium containing 100 microg/ml of erythromycin (EM). Among 11 EM-resistant strains that grew in the medium, point mutations in the 23S rRNA were found in 3 strains at A2063G, 5 strains at A2064G and 3 strains at A2064C. The relationship between the point mutation pattern of these EM-resistant strains and their resistance phenotypes to several macrolide antibiotics was investigated.