Seven of eight women who had given birth to infants with early onset or intrauterine infection caused by group B streptococci remained carriers of the same serotype of group B streptococci up to 38 months after their pregnancy. In contrast, only 34 of 88 group B streptococci carriers who had given birth to healthy infants harbored the same serotype at the 34 months' follow-up (P=.009). Among the control subjects, 29 of 71 showed increased serum levels at followup of antibodies against the serotype isolated at delivery, a significantly higher proportion compared with the mothers of infected infants/fetuses. The results indicate that mothers of group B streptococci-infected infants are chronic urogenital carriers of group B streptococci without responding immunologically against the organism.