The HCl-extracted purified polysaccharide type antigens of group B Streptococcus types Ia and Ib were composed of galactose and N-acetylglucosamine in a molar ratio of 3:1 for the Ia antigen and 2:1 for the Ib antigen. Immunochemical data were the same for the Ia antigens of type Ia, purified in this study, and type Ic, purified earlier. Glucosamine inhibited the Ib quantitative precipitin reactions more effectively than did N-acetylglucosamine, whereas the reverse was true of the Ia reaction. Ouchterlony studies were consistent with these observations and also revealed two type-specific precipitin bands with the HCl-extracted Ia antigens. All saline-extracted type antigens, however, formed single Ouchterlony bands that were only partially identical to the corresponding HCl antigens. Purification of the saline antigens was accomplished by treatment with cold trichloroacetic acid and by fractional ethanol precipitation. Immunoelectrophoresis experiments showed that the saline antigens were more negatively charged than the HCl antigens. The presence of sialic acid in the saline antigens probably accounted for their net negative charge and the fact that they were partially degraded by mild acid hydrolysis. The same serological specificities were observed with saline- and with HCl-extracted antigens.