The present invention describes derivatized polysaccharide-protein conjugates, a composition comprising one or more of such derivatized polysaccharide-protein conjugates and methods of immunizing human patients with the same. The derivatized polysaccharide-protein conjugates are purified capsular polysaccharides from Neisseria meningitidis serogroups A, C, W-135, and Y, derivatized, chemically activated and selectively attached to a carrier protein by means of a covalent chemical bond, forming polysaccharide-protein conjugates capable of eliciting long-lasting immunity to a variety of N. meningitidis strains
