Since Lancefield's classification of beta-hemolytic Streptococcus in 1933 (1), organisms designated Group B have been reported as a cause of human disease. Their emergence as frequent neonatal pathogens, however, is an event of this decade. Two distinct types of illness that are related to age at onset (2,3) are described for neonates with infections due to Group B Streptococcus. The "early onset type" of infection occurs in neonates 5 days of age or less (2), results from intrapartum transmission of organisms from the maternal genital tract, and causes a mortality rate in excess of 50%. "Late onset type" infections, in contrast, are characterized by onset beyond the 10th day of life and are associated with purulent leptomeningitis, significantly lower mortality rates, and modes of transmission in addition to the mother-to-infant route (4).These two clinical syndromes have been related to certain serotypes. The serologic classification of Group B streptococci is based upon capillary precipitin tests with hyperimmune rabbit antisera and acid-extracted antigens (5). By this method, Group B streptococci can be divided into serotypes Ia, Ib, Ic, II, and III (5-7). Serotyping of strains has allowed the epidemiologic characterization of neonatal infection. Organisms isolated from asymptomatic mothers and neonates with Group B streptococci have virtually identical distributions of serotypes; 26% of strains are type Ia, Ib, or Ic; 38% are type II, and 36% are type III (8). This distribution of serotypes is similar among isolates from neonates who have early onset type septicemia without meningitis. However, neonates who have early onset type infection with meningitis and infants with late onset type infection have a striking predominance of type III isolates (:>80%) (8-10). These epidemiologic observations suggested that strains of Group B streptococci of serotype HI may possess invasive properties that allow meningeal penetration and the production of disease beyond the first few days of life.Lancefield has immunochemically characterized two polysaccharides from Group B Streptococcus: the group-specific B polysaccharide antigen common to all strains and the type-specific polysaccharide antigen which allows classification into four distinct serotypes: Ia, Ib, II, and III (5,6). These type-specific petysaccharides are believed to be *