Levels of five antibiotics used in treating chorioamnionitis were measured in maternal and cord blood and placenta. Ampicillin provided the highest ratio of cord to maternal blood (0.71). Gentamicin was also relatively high in this respect and also had the highest placenta to maternal blood ratio (3.97). Based on these findings, the commonly used combination of ampicillin and gentamicin seems appropriate, although additional anaerobic coverage may be needed.