Because of the difficulty of conducting efficacy trials of vaccines against group B streptococcus (GBS), the licensure of these vaccines may have to rely on studies that measure vaccine-induced antibody levels that correlate with protection. This study estimates the level of maternal antibody required to protect neonates against early-onset disease (EOD) caused by GBS type Ia. Levels of maternal serum IgG GBS Ia antibodies, measured by ELISAs in 45 case patients (neonates with EOD caused by GBS Ia) and in 319 control subjects (neonates colonized by GBS Ia but without EOD) born at 34 weeks gestation were compared. The probability of developing EOD declined with increasing maternal levels of IgG GBS Ia antibody (P=.03). Neonates whose mothers had levels of IgG GBS Ia antibody 5 μg/mL had an 88% lower risk (95% confidence interval, 7%-98%) of developing type-specific EOD, compared with those whose mothers had levels <0.5 g/mL. A vaccine that induces IgG CaBS Ia antibody levels 5 μg/mL in mothers can be predicted to confer a high degree of type-specific immunity to EOD to their infants. © 2001 by the Infectious Diseases Society of America.