A monoclonal antibody (MAB) to the beta-1-6-linked digalactose structure in the lipopolysaccharide (LPS) of Bacteroides fragilis reacted with 47 of 416 group B Streptococcus (GBS) strains tested by an immunofluorescence technique (IF). The reactivity of MAB was, with a few exceptions, limited to type II GBS. Gas chromatography-mass spectrometry analysis demonstrated that an antigen purified by immunoaffinity chromatography using MAB from type II GBS contained galactose, glucose and fatty acids. This confirmed that MAB is directed to the digalactose (which in earlier studies was found to occur) in the capsular lipocarbohydrate specific to type II GBS. The positive strains yielded a strong, apple-green surface stain by means of the IF using MAB. Various immuno-electron microscopic (IEM) methods showed that the determinant was located in the glycocalyx layer of GBS at a distance of about 15 nm from the streptococcal cell wall. The structure harbouring the determinant was found to be very loosely attached to the bacteria. However the cross-reactive determinant seemed to maintain its immunoreactivity whether it was extracted by gentle washing with saline or with harsher treatments usually reserved for preparing streptococcal polysaccharide antigens. In conclusion, the study shows that the determinant is an integral part of the type-specific antigen of type II GBS and that MAB has a potential use as a serotyping reagent.