Colonization with group B streptococci of the genital tract was studied in 1 115 women during the last trimester of pregnancy. 76 or 6.82% were found to harbour this bacterium. The incidence of contamination was significantly higher among Belgian women than among parturients of Mediterranean origin (p less than 0.001). It was also more frequent in primigravidae (p less than 0.05) and in the poorer (0.10 less than p greater than 0.05). At the time of admission in the delivery room, it was noticed that rupture of the amniotic membranes for more than 24 hours was more often associated with group B streptococcal carriage by the mother (p less than 0.001). 29 out of 68 (42.6%) infants born to group B streptococci positive mothers were colonized at birth. 67 of them were submitted to a controlled trial of immediate versus delayed penicillin therapy. 44.8% and 42.1% of the neonates were contaminated at birth in each group of treatment respectively. No instance of group B streptococcal infection developed in either group. This suggests that immediate therapy with penicillin of infants of group B streptococci positive mothers has no definite advantage upon delayed treatment.