A total of 105 clinical strains of Group C and Group G streptococci were examined for their susceptibility to penicillin, cefotaxime, erythromycin, meropenem and vancomycin using a broth microdilution method. Minimum bactericidal concentrations of the antimicrobial agents and phenotypes of strains resistant to erythromycin were also evaluated. No resistance to penicillin, cefotaxime, meropenem and vancomycin was found in years 1995-2002, but there was 6.7% resistance to erythromycin. No tolerance was seen for penicillin and vancomycin, but there were strains tolerant to cefotaxime, erythromycin and meropenem. The resistance phenotypes of erythromycin-resistant isolates were determined by the double disc test with erythromycin and clindamycin which showed inducible MLS (57.1%) and M phenotype (42.8%) resistance. This in vitro finding shows that classical antimicrobial agents used for the treatment of GCS and GGS have good activity against clinically significant isolates, but the presence of macrolide resistance and tolerant isolates suggests that careful surveillance of the streptococcal isolates should be carried out. (C) 2003 Elsevier Science B.V. and the International Society of Chemotherapy. All rights reserved.