To reduce the number of injections needed to comply with paediatric vaccination requirements, a liquid, hexavalent vaccine (DTaP-IPV-PRP-T-HBs; Hexavac; Aventis Pasteur MSD) has been developed for primary and booster vaccination of infants and toddlers. In extensive clinical studies, Hexavac has been shown to be highly immunogenic. Seroconversion or seroprotective titres of antibodies against all antigens were achieved in the majority of infants following a primary series of three doses administered at 1-2-month intervals from 2 months of age. Hexavac also induced immunologic memory, as evidenced by the anamnestic response to booster vaccination at 12-18 months of age. These responses were comparable with those seen following concomitant administration of Pentavac (DTaP-IPV//PRP-T) and monovalent hepatitis B vaccine (H-B-Vax II), and were also within the ranges observed for other relevant licensed vaccines. Clinical studies comparing the immunogenicity of Hexavac administered at either 2, 3 and 4 months or 2, 4 and 6 months demonstrated that it can be used by either vaccination schedule. A further study also supported the use of primary doses of Hexavac at 3 and 5 months with a booster at 12 months of age. Hexavac demonstrated a good reactogenicity and tolerability profile. The most frequently reported adverse events after both primary and booster doses were local reactions of redness and swelling/induration and a systemic response of mild fever, irrespective of the vaccine used for priming. Hexavac provided immunity against six important childhood diseases with a single injection at each visit.