Intrapartum chemoprophylaxis with benzylpenicillin or erythromycin significantly reduced the rate of transmission of group B streptococci (GBS) from mothers colonized during pregnancy to their babies from 45% to 3% (P less than 0.001). None of the babies born to women who were given prophylaxis was colonized with GBS in the first 24 h of life. Six weeks after leaving hospital, however, 23% of the babies in the antibiotic group had become colonized with GBS compared with 44% in the control group. GBS strains resistant and tolerant to both benzyl-penicillin and erythromycin were found in this study. Intrapartum chemoprophylaxis breaks the cycle of GBS transmission at birth and may be useful in preventing early onset GBS disease, but is unlikely to affect late onset infections.