The present invention describes a combined vaccine that offers broad protection against meningococcal disease caused by the pathogenic bacteria Neisseria meningitidis. The vaccine is comprised of four distinct polysaccharide-protein conjugates that are formulated as a single dose of vaccine. Purified capsular polysaccharides from Neisseria meningitidis serogroups A, C, W-135, and Y are 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 in children as well as adults.