The susceptibility of neonates to invasive infection with type III group B streptococci (GBS) in a radioactive antigen-binding assay (RABA) has been correlated with low maternal serum levels of capsular polysaccharide-specific antibodies. An ELISA was developed using capsular polysaccharide covalently coupled to human serum albumin. In sera from 35 healthy women, the range of IgG antibodies to GBS III polysaccharide was 0.05-33.0 micrograms/mL, and specific IgA antibodies were 0.08-1.1 micrograms/mL; however, no GBS III capsular polysaccharide-specific antibodies of the IgM isotype were detected by the ELISA. The levels of naturally acquired and vaccine-induced antibodies obtained with this ELISA correlated well with the results of the RABA (Spearman's rank correlation coefficient, .92). The ELISA has two major advantages over the RABA: It measures specific isotypes and subclasses of antibodies, and it can detect type III polysaccharide-specific antibodies at lower concentrations.