There are only minor differences in the immunogenicity of the three Haemophilus b polysaccharide-protein conjugate vaccines licensed in the US when tested in children 17 to 19 months of age. In contrast, there are much greater differences in immunogenicity in 2-month-old infants. At this age, a single dose of PRP-OMPC evokes a strong primary antibody response, whereas repeated doses of HbOC or PRP-D are required to evoke an antibody response. These differences in immunogenicity are noteworthy, but they are not necessarily correlated with differences in the ability of different conjugate vaccines to confer protection against disease. Vaccination with all three of the conjugate vaccines primes infants for the ability to make a booster antibody response to reimmunization with unconjugated PRP vaccine and, possibly, to exposure to the encapsulated bacteria. Although unproven, this priming may be sufficient to confer protection against disease even in the absence of a 'protective' level of serum antibody.