The use of bacterial capsular polysaccharides as immunoprophylactic agents in human diseases caused by encapsulated bacteria is now firmly established (1). However, despite their many advantages, they do have serious limitations. First, they induce an inadequate immune response in infants (2), the section of the population most vulnerable to bacterial meningitis, and second, some polysaccharides are only weakly immunogenic in adults. To overcome these deficiencies, a new generation of semisynthetic vaccines have been developed based on the conjugation of polysaccharide to protein carriers.