Early-onset group B streptococcus (GBS) disease in the infant is acquired by vertical transmission from the mother colonized with GBS. Thirty-four women colonized with GBS were treated with intravenous ampicillin sodium during labor. None of their infants were colonized with GBS at birth or within 48 hours. Twenty-four women colonized with GBS received no antibiotic therapy; 14 (58%) of their infants were colonized with GBS at birth or by 48 hours. This difference was highly significant. Mechanisms by which this may have occurred were temporary suppression of GBS vaginal and rectal colonization, high concentration of ampicillin in the amniotic fluid, and transplacental transport of the antibiotic to the infant. In areas where GBS disease is prevalent, we recommend screening pregnant women (34 to 36 weeks' gestation) and treating those colonized with GBS (with no history of penicillin hypersensitivity) with intravenous ampicillin during labor.