The present invention generally provides compositions comprising a polysaccharide derivative, and methods of their preparation and use for the prevention or treatment of diseases caused by Neisseria meningitidis bacteria, particularly group B (NmB) strains, and by E. coli K1. The invention provides a de-N-acetylated PS derivative in which one or more residues of the PS has been modified by de-N-acetylation. The invention also includes derivatives in which one or more of the N-acetyl groups of PS containing de-N-acetylated PS are replaced with other N-acyl groups, usually a lower acyl group of C.sub.2-C.sub.3. Further, the invention includes de-N-acetylated PS derivatives containing long chain hydrocarbons, as well as conjugates in which the de-N-acetylated PS derivative is linked to a carrier, e.g., a carrier protein.