GBS is the leading cause of perinatal bacterial infections in the United States. Selective intrapartum chemoprophylaxis can prevent GBS early-onset neonatal disease and reduce maternal puerperal morbidity. There does not appear to be an effect on late-onset neonatal disease. Protocols for intrapartum antibiotic chemoprophylaxis based on antepartum cultures, testing at parturition, and clinical risk factors have been summarized. While screening of all pregnant women for GBS is an option, screening and selection methods need further development. A highly reliable rapid screening test for intrapartum detection of light and heavy GBS colonization of the lower genital tract is not yet available. Intrapartum antibiotic chemoprophylaxis of GBS carriers with one or more risk factors substantially reduces the frequency of GBS disease. Benefit may also be obtained from prophylaxis based solely on a knowledge of risk factors if GBS carrier status is unknown. The importance of GBS infection as a perinatal problem and its considerable economic burden justify implementation of chemoprophylactic programs by obstetricians, particularly those who encounter a high proportion of patients with perinatal risk factors in their practices.