The aim of this study was to examine the immunogenicity of a Haemophilus influenzae type b (Hib) vaccine (PRP-D) in Danish children younger than 18 months of age, given at the same time as a diphtheria-, tetanus- and polio-vaccine at the age of five, six and 15 months. The study was carried out as a multicentre study, where 21 children were vaccinated by their general practitioners. Blood samples were drawn just before and four weeks after each of the three vaccinations and the concentration of Hib-antibodies were determined using an ELISA-technique. Before the first vaccination none of the 21 children had antibody levels above 1 microgram/ml, which is believed to be the level which provides long-lasting protection, whereas after the first, second and third vaccination respectively 24, 48 and 100% of the children had antibody levels above 1 microgram/ml. There were only mild and short-lived side effects. The PRP-D vaccine given at the same time as Di-Te-Pol is safe, without serious side-effects and immunogenic in Danish children.