Concentrations of ampicillin were measured in maternal serum at delivery, in cord serum, and in neonatal serum sampled at 1, 4, and 8 hours after ampicillin had been administered prophylactically to 22 mothers who were undergoing cesarean section. The concentrations of ampicillin in maternal serum at delivery ranged from 4.6 to 50 micrograms per milliliter and were inversely related to the time between administration of the drug and delivery. Concentrations in cord serum ranged from 4.4 to 23 micrograms/ml and were lower than those in corresponding maternal samples in all but two cases. The ratios of cord/maternal concentrations of ampicillin ranged from 0.16 to 1.3 and were directly related to the time which had elapsed between administration of ampicillin and delivery. The mean concentration of ampicillin in cord and neonatal serum declined exponentially in the 8 hours after infusion; it exceeded the minimal inhibitory concentration against Escherichia coli for 4 hours and against Listeria monocytogenes and group B streptococcus for at least 8 hours.