OBJECTIVE: to determine the in vitro susceptibility of group B streptococci (GBS) to antibiotics, used for intrapartum chemoprophylaxis and treatment of infections; to determine the rate of resistance to erythromycin and clindamycin and the phenotype distribution of GBS strains. METHODS: 262 GBS strains from pregnant women at 35-37 weeks' gestation were collected in University Hospital Gasthuisberg (Leuven) and Imelda Hospital (Bonheiden). The minimum inhibitory concentrations (MICs) of penicillin G, amoxicillin, cefazolin, cefotaxime, erythromycin, clindamycin, gentamicin, vancomycin and linezolid were determined by the agar dilution method, according to NCCLS guidelines. RESULTS: all isolates were susceptible to penicillin, amoxicillin, cefazolin, cefotaxime, vancomycin and linezolid. We found resistance rates of 16.7% to erythromycin and 11.0% to clindamycin. Of all erythromycin-resistant strains, 63.6% had the cMLSB phenotype, 20.5% the iMLSB phenotype and 15.9% the M-phenotype. For 25% of erythromycin-resistant strains, the resistance was of a very high level (MICs ranging from 128 microg/mL to 256 microg/mL). All these isolates belong to the cMLSB phenotype. For the remaining 75% the resistance to erythromycin was of low level (MICs ranging from 1 microg/mL to 4 microg/mL). These isolates had the cMLSB phenotype (38.6%), the iMLSB phenotype (20.5%) and the M-phenotype (15.9%). CONCLUSION: the susceptibility of GBS to the beta-lactam antibiotics supports the continued use of penicillin for intrapartum chemoprophylaxis. For women who are allergic to penicillin, clindamycin or erythromycin are considered to be the alternatives, however resistance rates to these antibiotics are significant.