Seventy-six (13.6%) of 558 third trimester pregnant women had positive vaginal Group B Streptococcal (GBS) colonization. Among these colonizing GBS, 17 (22.4%) of 76 reacted positively to type Ia serum, 6 (7.9%) to type Ib serum, 2 (2.6%) to type II serum, and 23 (30.3%) to type III serum respectively, whereas 22 (28.9%) lacked these heat stable antigens when examined by the agglutination method. Persistence of the colonization of GBS was observed only in 4 (9.9%) out of 44 ampicillin-treated women whereas 65 (86.7%) out of 75 cases in the non-treated group. The colonization rates of GBS in infants born to carrier mothers were 37.5% in the non-treated group and 7.4% in the ampicillin treated group. The levels of ampicillin concentration in cord sera and amniotic fluids were maintained over the MIC of GBS for 7 hours after the drug administration. Type-specific antibodies were assayed by indirect immunofluorescence in both maternal and cord sera. In 58(87%) of 67 cases antibodies to type Ia, in 15(27%) of 55 cases to type II, and in 28(42%) of 67 cases to type III were detected in the maternal sera. No antibody to type Ia was detected in a mother who had an affected infant with the same type of GBS. In cord sera, antibody titres of a premature infant and a low-birth-weight infant were lower than those of their mothers.