Background.Despite their proven efficacyHaemophilus influenzaetype b (Hib) conjugate vaccines are not given to most children in the developing world in the face of an estimated global Hib disease burden of nearly 2 million cases per annum. A major barrier to the introduction of the vaccine would be overcome by diluting the vaccine 10-fold in diphtheria-tetanus-whole cell pertussis (DTP). We report a randomized trial comparing the use of Hib conjugate vaccine diluted in a multidose vial of DTP with that of the full Hib dose.Methods.We randomized 168 infants to receive either the full dose Hib polysaccharide-tetanus toxoid conjugate (PRP-T) vaccine or a 1/10 dilution prepared by reconstituting the full dose in a 10-dose DTP vial. Infants were vaccinated at 6, 10 and 14 weeks of age and received a full dose as a test of immunologic memory at 9 months of age. Sera were collected at each visit and at 1 week after the booster dose. Serum anti-capsular PRP antibody concentrations were measured by enzyme-linked immunosorbent assay.Results.After the primary vaccination series, 95% of infants in the full dose arm and 94% of infants in the 1/10 dose arm achieved anti-PRP IgG antibody concentrations of ≥1.0 &mgr;g/ml. Infants receiving the diluted vaccine had significantly higher titers of anti-PRP antibody in response to the booster dose (151.36 &mgr;g/mlvs.68.55 &mgr;g/ml,P= 0.009).Conclusions.The 1/10 dose of PRP-T was as immunogenic and safe as the full dose. The technique of diluting a single dose of PRP-T in a 10-dose DTP vial could potentially allow the widespread introduction of Hib vaccine in resource-poor countries currently unable to afford full dose Hib conjugate vaccine.