Of 858 pregnant women studied in matched rectal, urethral and urine cultured specimens, 186 (22%) were found to be colonized by group B streptococci (GBS). GBS were detected significantly more often in rectal specimens (159) than in urethral specimens (108) or in urine specimens (64). This is supporting evidence for the gastrointestinal tract as the main habitat of GBS. Of 1786 women whose urine was sampled at delivery, GBS were isolated from 128 (7%), in 22 of whom (1% of the total) GBS were present in quantities greater than or equal to 10(4) colony forming units (cfu)/ml urine. Neonates born to women with greater than or equal to 10(4) cfu GBS/ml urine were apparently at greater risk for neonatal infection, as they were more commonly and more heavily colonized than were the newborns of women with lower quantities of GBS in urine, or if positive urethral or rectal specimens were considered. The incidence of preterm delivery or obstetric infection was not higher among women in whom GBS were isolated in specimens from any of the 3 sites; foetal distress was more common among their children, but not neonatal respiratory or infectious diseases of which the incidence was low and difficult to assess statistically.