A randomized study in 121 pregnant women carrier of group B streptococci is undertaken in order to assess the administration of 500 mg of intrapartum ampicillin intravenously to interrupt mother-to-fetus group B streptococcal transmission. In the prophylaxis group there was a significant reduction in neonatal colonization (3.7 vs. 42.9%) and in severe neonatal colonization (0 vs. 25%). There was no case of group B streptococcal sepsis in the prophylaxis group compared to 4.6% (3 cases) in the control group (P greater than 0.05). Clinically infected newborns represented 3.3% in the prophylaxis group vs. 13.8% in the control group. When the organism was isolated during delivery in the vagina or amniotic fluid, prophylaxis was quickly followed by second negative cultures. Ampicillin levels in the amniotic fluid were detected early, and they increased significantly till the third hour. Bactericidal levels in the umbilical cord were detected in 60% of newborns. All these findings support the usefulness of ampicillin prophylaxis in the prevention of early-onset group B streptococcal sepsis.