Immunogenicity data obtained after primary series immunisations against Haemophilus influenzae type b (Hib), using a vaccine prepared by conjugating the capsular polysaccharide of Hib to tetanus toxoid (ActHIB), were compiled from 146 study groups comprising 85 clinical trials or vaccination programs conducted between 1987 and 1999. ActHIB was administered as a monovalent lyophilised vaccine, injected either in association with another paediatric vaccine (at separate administration sites) or in combination (where the different vaccines are mixed together in the same syringe before injection). Review of these data reveals two trends. First, PRP-T vaccine, given either alone or in combination with DTwcP, resulted in a stronger anti-PRP serum antibody response than when PRP-T was combined with DTacP vaccine. Second, an accelerated (i.e. one-month interval) immunisation schedule tended to induce a poorer anti-PRP antibody response than did the more widely spaced, standard inoculation schedules. Although an in-depth analysis of these over 11000 study subjects on an individual basis with multivariate analysis or multifactorial statistical methods might eventually provide working hypotheses to fully understand these phenomenon, the various licensed, PRP-T-containing paediatric combination vaccines have proved to be clinically effective.