152 women were cultured for group B streptococci (GBS) weekly from the 37th week of gestation and at admission to hospital for delivery. Matched rectal, urethral and urine specimens were collected for study (mean 4 times). In the 37th week of gestation, 33 women (22%) harboured GBS in rectal specimens, 28 women (18%) in urethral specimens, 16 women (11%) in urine specimens, and 37 women (24%) in at least one of the 3 specimens. All cultures considered, a total of 46 women (30%) yielded GBS in at least one culture. In the 37th week of gestation, women subsequently found to be GBS colonized at labour (positive in at least one site) had a higher rate of positive cultures in rectal specimens (77%) than in either urethral (67%) or urine specimens (41%). Chronic GBS carriage was more frequent in rectum than in urethra or urine. The results of the present investigation support the gastrointestinal tract as being the predominant source of GBS.