Streptococcus agalactiae (group B streptococcus; GBS) organisms are a major cause of severe infections, including bacteremia and meningitis in newborns. According to previous reports, GBS organisms are uniformly sensitive to penicillin G (PCG). The susceptibility of 117 strains isolated at Yodogawa Christian Hospital in Osaka, Japan, in 2001 was examined with the WalkAway system, using currently valid National Committee for Clinical Laboratory Standards (NCCLS) interpretive criteria. Twenty-one strains (18%) had intermediate susceptibility and 1 strain (1%) was resistant to PCG. Fifty-one strains (44%) had intermediate susceptibility to ampicillin (ABPC). No ABPC-resistant strain was found. Six GBS strains were selected from the 51 strains showing intermediate susceptibility to ABPC to determine the minimal inhibitory concentrations (MICs). The MICs of the 6 strains were: 1 microgram/ml to ABPC, 0.25 microgram/ml to PCG, 2 micrograms/ml to cefotaxime (CTX), 0.016 microgram/ml to panipenem (PAPM), and more than 4 micrograms/ml to erythromycin (EM). These 6 strains were distinctly resistant to CTX. Peak concentrations in excess of three to ten times the bactericidal concentrations at the site of infection are associated with the best clinical response. In meningitis caused by GBS whose susceptibility is intermediate or resistant to PCG or ABPC, it is difficult to maintain a sufficient therapeutic concentration in cerebrospinal fluid after the administration of these two agents. It is preferable to use PAPM, because the efficacy and safety of PAPM in the treatment of purulent meningitis caused by penicillin-resistant Streptococcus pneumoniae were established in Japan.