Background :Recent data suggest that the colonization rate of group B streptococci (GBS) in pregnant women and the incidence of neonatal infections by GBS is increasing trend in Korea, but the antimicrobial susceptibilities and serotypes in pregnant women have not been reported in Korea. So, we studied to define the antimicrobial susceptibility patterns and frequency of serotypes of GBS in pregnant Method :The susceptibility and serotyping of 60 GBS isolates from 27 pregnant women and four isolates from their two neonates were tested by an agar dilution method and agglutination test, respectively. The typing sera used in this study were Ia, Ib, II, III, IV, and V. Result :Minimal inhibitory concentration range of 60 GBS from pregnant women were penicillin G 0.015-0.12 ug/ml, vancomycin 0.5-2 ug/ml, clindamycin 0.015-4.0 ug/ml, chloramphenicol 2-4 ug/ml, erythromycin 0.015-2 ug/ml, tetracycline 0.5-256 ug/ml, cephalothin 0.12-0.25 ug/ml, ceftriaxone 0.03-0.12 ug/ml, respectively. The resistance rate of GBS were 6.7% to clindamycin, 0% to erythromycin, and 98.3% to tetracycline. Most of GBS serotypes from pregnant women in decreasing order were Ib (48.3%), Ia (24.1%), III (20.7%), Conclusion :All GBS strains isolated from pregnant women are highly susceptible to commonly used antimicrobial agents with the exception of tetracycline. The low prevalence of severe neonatal GBS infections in Korea is not due to the absence of serotype 111, but probably due to a low genital carriage rate of GBS by pregnant women

Keywords: Group B streptococci (GBS), Pregnant women, Serotype, Minimal inhibitory concentration