Two monoclonal mouse antibodies with specificities for group B streptococcal capsular antigens were evaluated in assays for the identification of group B streptococci (GBS). One of these antibodies (A9) was shown to precipitate group B carbohydrate antigen in reactions with both purified group B antigen and antigen present in autoclave or enzyme extracts of GBS. A9 antibody was also specific for group B antigen in gel diffusion reactions with extracts of Lancefield group A, B, C, D, F, and G streptococci and was a highly sensitive reagent in detecting soluble group B antigen by counterimmunoelectrophoresis. Antigen extracted from all five serotypes of GBS was shown to be precipitated by A9 antibody. A second monoclonal antibody (C8) was reactive with intact GBS but did not precipitate soluble antigen in bacterial extracts. In contrast with what has been shown for polyclonal rabbit anti-group B antiserum, neither antibody was significantly inhibited in binding or precipitation assays by high concentrations of free rhamnose or other monosaccharides of carbohydrates found in group B antigen. Rhamnose, the most abundant carbohydrate of the group B antigen, does not appear therefore to be an immunodominant determinant in the binding of A9 or C8 antibody. The epitopes of both monoclonal antibodies are exposed on the surface of live as well as heat-fixed GBS cells. A9 antibody-coated latex particles were compared with a commercially available polyclonal latex agglutination reagent and shown to be equally sensitive and specific in the detection of soluble group B antigen in urine and cerebrospinal fluid from patients with GBS infections. Because of its uniformity and defined antigen specificity, which includes reactivity with all five serotypes of GBS, A9 antibody offers the potential of an improved immunodiagnostic reagent for the identification of GBS.