PedvaxHIB, a Haemophilus influenzae type b (Hib) conjugate vaccine composed of Hib capsular polysaccharide covalently bound to an outer membrane protein complex of Neisseria meningitidis serogroup B, was evaluated for immunogenicity and safety in infants and children 2 months of age and older. A significant and consistent antibody response was seen after a single dose of the vaccine in all age groups, including infants as young as 2 months of age. In addition, the vaccine elicited a good booster response when given at 12 to 17 months of age. Subjects from diverse subpopulations, including those with impaired antibody response to Hib polysaccharide vaccines, showed a significant response to vaccination. The vaccine was well tolerated when administered alone or concurrently with other paediatric vaccines. A protective efficacy study, recently completed, has shown the vaccine to be highly effective in 2-month-old infants.