Antibodies to the capsular polysaccharides of Haemophilus influenzae type b or Streptococcus pneumoniae protect against meningitis caused by these bacteria. Many of the polysaccharides are poorly immunogenic, especially in infants, but can be turned to highly immunogenic vaccines by covalent conjugation to a protein carrier. On the basis of the good protection observed in several trials, H. influenzae type b conjugates have been accepted for wide use. This experience has also provided direction for the development of new conjugates against infections caused by the most common serotypes of S. pneumoniae. First results from immunogenicity studies of these pneumococcal conjugate vaccines are promising.