The pharmacokinetics of amoxicillin were studied in umbilical cord and neonatal sera relative to maternal concentrations in prevention of neonatal group B streptococcus infection. The subjects were 44 pregnant women receiving amoxicillin as 1 or 2 g as an intravenous infusion. To measure the concentrations, blood samples were obtained from the mother, the arterial and venous umbilical cord, and the neonate. The pharmacokinetics were characterized by a five-compartment model by using nonlinear mixed-effects (population) modeling. The population estimates for the clearance, central volume of distribution, and the two peripheral maternal volumes of distribution were 19.7 +/- 0.99 liters/h, 6.40 +/- 0.61 liters, and 5.88 +/- 0.83 liters (mean +/- standard error), respectively. The volume of distribution of the venous umbilical cord and the neonatal volume of distribution were 3.40 liters and 11.9 liters, respectively. The pharmacokinetic parameter estimates were used to simulate the concentration-time profiles in maternal, venous umbilical cord, and neonatal sera. The peak concentration in the venous umbilical cord serum was 18% of the maternal peak concentration. It was reached 3.3 min after the maternal peak concentration. The concentration-time profile in neonatal serum was determined by the profile in venous umbilical cord serum, which in turn depended on the profile in maternal serum. Furthermore, the simulated concentrations in maternal, venous umbilical cord, and neonatal sera exceeded the MIC for group B streptococcus for more than 90% of the 4-h dosing interval. In a first approximation, the 2-g infusion to the mother appears to be adequate for the prevention of group B streptococcal disease. However, to investigate the efficacy of the prophylaxis, further studies of the interindividual variability in pharmacokinetics are indicated.