Amoxicillin is effective against S. agalactiae (MIC 50 = 0.03 mg/l). Neonatal prophylaxis of GBS infection had already been studied but very few data are available regarding pharmacokinetic of these antibiotics. In this study, 58 pregnant women with GBS colonisation and/or with epidemiologic risk factors received intrapartum antibiotic infusion at the beginning of the labor (1 g amoxicillin every 6 hours until the delivery). At delivery, cord blood and gastric fluid were taken in delay from 0.5 to 6 hours after the beginning of the infusion. Dosages were done in triplicate with a microbiological method using Bacillus subtilis ATCC 6633 as test strain. In cord blood amoxicillin appeared as early as the first half hour. Concentration were from 5 to 7 mg/l between 1 hour and 4 hours after the beginning of the infusion, and from 3 to 4 mg/l after 4 hours. In gastric fluid, concentrations obtained were over 1 mg/l after 1.5 hour and over 3 mg/l after 2.5 hours with good efficiency even after the sixth hour. The protocol using a slow intrapartum amoxicillin infusion (1 g) gave concentrations in gastric fluid and cord blood over S. agalactiae MIC.