In the USA, three bacteria polysaccharide vaccines are currently available. These are: (i) Haemophilus influenza type b; (ii) memingococci belonging to serogroups A, C and Y; and (iii) a multivalent vaccine for Streptococcus pneumonia (pneumococcus). These bacteria are primarily causative organisms of acute diseases of infancy and a problem relating to the use of such polysaccharide vaccines is the poor response elicited in young children under the age of 2 years, the group who are often most at risk from the infections caused by these bacteria. Attempts are therefore frequently made to improve the antigenicity of the polysaccharides through coupling to proteins or other chemical modification. Control of these serogroups of meningococci is however now possible, at least in the short term, through the use of purified exopolysaccharide material in vaccines. The H. influenza type B strains are also causative organisms of meningitis. These highly virulent bacteria form one of six antigenically and chemically distinct capsular chemotypes. Although antipolysaccharide antibodies provide protection against infection, the polymer is poorly antigenic and does not induce protective immunity in infants of two years old or less (the most susceptible group to H. influenza meningitis). Immunogenicity can be enhanced through coupling to proteins; it has been suggested that a protein from the outer membrane of the causative microorganism should be used. The capsular polysaccharide is a linear heteropolymer composed of alternating units of ribose and ribitol linked by a phosphodiester group, which has proved amenable to coupling to protein.