Group B streptococci (GBS), Streptococcus agalactiae, are Gram-positive bacteria occurring in pairs and short chains. S. agalactiae was originally investigated as a frequent cause of bovine mastitis; before the widespread use of penicillin, it accounted for as much as 90% of all bovine mastitis. It was first isolated from parturient women in 1935. Since the 1970s, GBS have emerged as important human pathogens and have remained the major cause of bacterial diseases in newborns in the western world. In 1990, there were approximately 7600 neonatal cases of invasive GBS disease in the USA. Since then, intense intrapartum use of antibiotics has decreased the incidence of neonatal early-onset GBS disease, to 2200 cases in 1998. Babies can be exposed to GBS during labour and delivery, when they swallow or inhale the bacteria while passing through the birth canal of the colonized mothers. Babies can also be exposed in utero. GBS infection in infants causes sepsis and meningitis, which can result not only in illness and death but also in long-term disabilities such as hearing loss, impaired vision, developmental problems and cerebral palsy. The direct costs of neonatal disease alone in the USA have been estimated at 300 million dollars annually. GBS is also an important cause of maternal illness and a risk factor for preterm delivery.