Abstract: The invention relates to chemically-modified group B polysaccharides of Neisseria meningitidis. The invention also provides vaccines in which the respective modified polysaccharides are conjugated to a protein carrier, and the like. More specifically, the present invention provides novel group B meningococcal unsaturated N-acyl derivative polysaccharides, novel conjugates of the group B meningococcal unsaturated N-acyl derivative polysaccharides, pharmaceutical compositions comprising conjugate molecules of group B meningococcal unsaturated N-acyl derivative polysaccharide fragments covalently bound to proteins, and the use of these compositions as vaccines.