In a population of pregnant women, the prevalence of group B streptococcal carriage was relatively low. During the 3rd trimester of pregnancy 5.6% of women haboured group B streptococci and 8.3% were positive at the onset of labour. Some 42% of women who gave positive cultures in labour had given negative cultures during the 3rd trimester and 19% of women who were positive during late pregnancy were culture negative in labour. The conversion of culture status observed in these women suggests that carriage may be intermittent or that new acquisition of genital tract streptococci may occur in late pregnancy. The unpredictability of conversion diminishes the relability of a single culture taken during the 3rd trimester of pregnancy. Acquisition of streptococci was seen in nearly 50% of infants born to women who were culture positive in labour. The maternal site of carriage (cervix or vagina or both) did not appear to influence the likelihood of bacterial acquisition by the infants. The isolation of streptococci from multiple sites was quite common in the infants, and the intensity of bacterial contamination was usually high. The external ear canal was a favourable site for detecting neonatal contamination with group G streptococci; 94% of infants, who gave positive cultures at birth had positive ear canal cultures, and in 26% the ear canal was the only site from which streptococci were isolated. This suggests that bacterial contamination of amniotic fluid by the ascending route was a common event in culture positive women. Three infants apparently had asymptomatic streptococcal bacteriaemia. Isolation of the same serotype of group B streptococcus from the infants and their mothers, and the presence of the organism in the infants, often in large numbers, shortly after birth, support the view that the organism is usually transmitted from the maternal genital tract to the infant. There was no evidence that any one type was more likely than others to be transmitted from mother to infant. Group B streptococci of serotypes Ic, II, and III were the predominant ones in the infants and their mothers in this study.