The immunochemistry of capsular type polysaccharide and virulence characteristics of group B streptococci (GBS), type VI, were studied. By high-pressure anion-exchange chromatography and pulsed amperometric detection, as well as by 13C nuclear magnetic resonance analysis, both extracellular and cell-bound polysaccharides were found to contain glucose, galactose, and N-acetylneuraminic acid in the molar ratio of 2:2:1, respectively. At variance with all other GBS serotypes described to date (Ia, Ib, II, III, IV, and V), no N-acetylglucosamine was present, whatever the source of the material (secreted or cell bound; reference or clinical isolate). Sialic acid was probably involved in the immunodeterminant structure of this new serotype since cleavage of this sugar from the polysaccharide gave rise to an antigen which reacted very weakly with type VI antiserum and to a precipitation line in immunodiffusion with no identity with the native type VI polysaccharide. By using type VI antiserum and the protein A-gold technique, a large capsule was observed in the type VI GBS reference strain by electron microscopy. All type VI strains examined were lethal for CD-1 mice, the 50% lethal dose after intraperitoneal challenge ranging from 1.0 (+/- 0.9, standard deviation) x 10(5) to 2.5 (+/- 1.5, standard deviation) x 10(5) CFU per mouse. A rabbit antiserum against capsular type polysaccharide exhibited both protective activity for mice injected intraperitoneally with type VI reference strain or with clinical isolates and opsonic activity in a phagocytosis assay.