1. Among group B hemolytic streptococci one serological type previously described as homogeneous has been shown, instead, to contain two closely related types, distinguishable by reciprocal absorption experiments. These streptococci are designated Types Ia and Ib.
2. Homologous organisms in each case absorb all antibodies from their respective antisera, while organisms of the heterologous related type absorb only the antibody responsible for the cross reactions. Group B streptococci of other types do not absorb the antibodies responsible for the cross reactions between these two related types. The precipitin reaction and passive protection tests in mice were employed in this analysis.
3. The type-specific substance of Type Ia is a polysaccharide. Preliminary study indicates that this is also true of Type Ib. While no data are available concerning the chemical relationships of these substances, it seems probable that the two types elaborate polysaccharides, related chemically as well as serologically.