Urethral and cervical specimens were obtained from 786 parturients during labour. 126 women (16%) were found to be urogenital carriers of group B streptococci (GBS). Bacteriological specimens were also obtained from the throat, umbilicus and external auditory canal of their 786 infants immediately after birth, and from 671 of the infants staying at the maternity unit 4 days later. 51% (64/126) of the infants born to GBS carriers were culture-positive for GBS immediately after birth. Only 27% (6/22) of the infants born to women who were GBS culture-positive in the urethra but not in the cervix contracted GBS at the delivery, in contrast to 59% (58/85) of the infants born to combined urethral and cervical carriers (P less than 0.05). This difference in colonization rate was not related to differences in levels of antibodies to the type of GBS carried by the individual parturient. On day 4, 13% (90/671) of the infants in the maternity unit were colonized with GBS. 39% of them were colonized at birth from their mother, 12% were culture negative at birth but had become colonized by day 4 with the same type of GBS as that isolated from the urogenital tract of the mother, and 37% were GBS positive on day 4 but culture-negative at birth and the mother's specimens did not reveal GBS. The distribution of serotypes among these infants was identical with that found among the other colonized infants, indicating that they might have contracted their GBS from the other infants in the maternity unit.