We report the first characterization of group B meningococcal polysaccharide a t 28OC in 0.4M KC1 and 0.05M sodium acetate at pH 5.61 and in pure water by means of light-scattering spectroscopy. In buffer solution, the weight-average molecular weight M, = 1.46 X lo5 g/mol, the 2-average radius of gyration (rg2)f'* = 26.3 nm, the second virial coefficient A2 = (2.12 f 0.17) X cm3 mol/g2, and the second virial coefficient of the diffusion expansion kD = 79 g/g. By using a histogram approximation on the correlation function profile analysis, we observed size fractions of two sizes with ?hL/?hs = 298 8,/50 8, and ALIAS = 82/18, where 6, and A are the respective effective hydrodynamic radius and the integrated intensity and the subscripts L and S denote the large and small size fractions. In pure water, ? h L / k = 402 A/29 8, with ALIAS = 84/16. The linewidth results are consistent with the measurements on the angular distribution of scattered intensity. Furthermore, if we take the large fraction to behave more like solid spheres, we are able to determine a molecular weight distribution of B-polysaccharide in buffer solution in a bimodal size distribution function.