OBJECTIVE: To test the antibiotic susceptibility and study mechanisms of macrolide resistance in group B streptococcus isolates (GBS). METHODS: The GBS investigated in this study included 140 and 47 colonizing strains isolated from vaginal or cervical swabs from pregnant women in Beijing (from 1994 to 1999) and Guangzhou obstetrics and gynecology hospitals (from 1999) and 6 invasive strains isolated from infants in Beijing Children's Hospital. Susceptibility to ampicillin, penicillin G, erythromycin, lincomycin, cephazolin, cefuroxime, cefoperazone was assessed by K-B disc diffusion. The mechanisms, methylation or efflux, of macrolide resistant GBS isolates, were analyzed by PCR for ermB and mefA genes. RESULTS: Susceptibility testing revealed that none of the GBS isolates were resistant to beta-lactam drugs, but 17% isolates showed intermediate susceptibility to penicillin G and ampicillin. The rate of erythromycin resistance increased from 8% in 1998 to 16% in 1999 in Beijing, while the rate of lincomycin resistance increased from 20% to 28% in that period. However, 21 (45%) and 12 (26%) isolates were resistant to erythromycin and lincomycin, respectively in Guangzhou city where erythromycin resistance rate was higher than that in Beijing. Of 45 erythromycin resistant isolates, 20 (20/45, 44%) possessed the ermB gene and 13 (13/45, 29%) harbored the mefA gene; 6 isolates harbored both genes, 6 isolates had possessed neither ermB gene nor mefA gene. CONCLUSION: The susceptibility of GBS isolates to penicillin G and ampicillin suggests use of penicillin G or ampicillin as a first-line drug in prophylactic treatment regimes against early-onset neonatal GBS disease. Erythromycin and lincomycin should not be recommended as the second-line antimicrobial in Beijing and Guangzhou city. The clinical relevance of macrolide resistant GBS in women treated with macrolides for intrapartum prophylaxis needs to be assessed. Ribosomal modification by a methylase encoded by erm gene may play a major role in the mechanisms of macrolide resistance of GBS isolates in China.