Since 1970 there has been an increase in isolations of Group B beta-haemolytic streptococci from infants and mothers at the National Women's Hospital and the organism has become the major cause of fatal perinatal infection. Forty-three of 60 stillborn and liveborn infants with postmortem isolations of Group B streptococci had pneumonia and of these a minority also had meningitis and/or septicaemia. Amnionitis was found in 15 of 20 placentae examined from these patients and an ascending infection from the maternal genital tract, often through intact membranes, was considered likely in the majority. However, a review of the prenatal histories of 33 infants showed that only a minority had premonitory features such as prolonged rupture of membranes, prolonged labour or maternal fever. Thirteen of 26 liveborn infants had a birth weight less than 2500 g. The majority presented within one hour of birth with respiratory distress or apnoea and died within 48 hours of birth. Early diagnosis of Group B infection is possible if bacteriological and radiological evidence is sought in infants of low birth weight, with low Apgar scores and with early onset of respiratory distress syndrome or apnoea in addition to those having the more usual indications of intrauterine infection. Group B streptococci were carried vaginally in 9 per cent of women attending an antenatal clinic and this high carrier rate is considered to preclude prophylactic treatment.