Early-onset group B streptococci (GBS-EOS) sepsis may be prevented by intrapartum antibiotics administered for GBS maternal colonization, premature labor, or prolonged rupture of membranes. We sought to identify cases of neonatal GBS sepsis after apparent failure of intrapartum chemotherapy and to determine the factors associated with failure of intrapartum antibiotics in these cases. We identified 96 GBS blood culture-positive infants at five military medical centers from 1987 to 1990. Eighteen (18.7%) of these infants had mothers who had received intrapartum antibiotics; 16 of 18 cases were early-onset disease, 15 of which initially had symptoms at less than 1 hour of age. Two infants had late-onset disease develop at 3 weeks of age. At least one perinatal risk factor (prematurity, prolonged rupture of membranes > 12 hours, maternal fever) was present in each of the 16 cases. Indications for intrapartum antibiotics were suspected chorioamnionitis (13 cases), GBS colonization and prolonged rupture of membranes or prematurity (3), and GBS colonization alone (2). Maternal antibiotics included ampicillin (14 cases), cephadyl (1), vancomycin (1), clindamycin (1), and gentamicin alone (1). The median number of doses of ampicillin before delivery was 1 (range, 1 to 21), which was administered at a median of 4 hours (range, 1 to 84) before birth. The mean dose of ampicillin was 1.8 gm/dose (range, 1 to 2 gm/dose). Two of 16 (12.5%) infants with GBS-EOS died as a result of GBS sepsis. In our population of neonates with GBS-EOS, 18.4% (16 of 87) of the infants had positive blood cultures despite intrapartum antibiotics. Intrapartum antibiotics may fail to prevent GBS sepsis in a number of infants born to mothers colonized with GBS or to those with acute chorioamnionitis.