Mouse monoclonal antibodies were used in the immunochemical characterization of the polysaccharide antigens of group B streptococci (GBS). Monoclonal antibodies specific for the sialylated form of the GBS type III polysaccharide were highly protective in a mouse model of GBS type III infection, whether of IgM, IgG2a, or IgA isotypes, but monoclonal antibodies specific for the nonsialylated form of the type III antigen were not protective, regardless of isotype. Monoclonal antibodies reacting with the type II polysaccharide antigen could be divided into two general antigen binding groups on the basis of the ability of beta-methylgalactopyranoside to inhibit their binding to purified type II antigen. Various proportions of antibodies with the two specificities were observed in rabbit and human sera. Although it was previously reported that rabbit antisera could not distinguish between the sialylated and nonsialylated forms of the type Ib polysaccharide, mouse monoclonal antibodies were found to exhibit exclusive specificity for one or the other form of the antigen. Only monoclonal antibodies specific for the sialylated Ib polysaccharide were protective in a mouse model.