An open randomised controlled multicentre study compared the immunogenicity and reactogenicity of three vaccines given by injection at two, three and four months of age. Children (89) received Haemophilus influenzae type b (Hib) vaccine (SmithKline Beecham Biologicals [SB]) administered in the same syringe with combined diphtheria-tetanus-whole-cell pertussis (DTPw) vaccine (Evans); 75 received Hib vaccine (SB) administered as a separate injection with DTPw vaccine; 66 received Hib vaccine (Pasteur Merieux [PM]) administered as a separate injection with DTPw vaccine. All subjects in both groups receiving Hib (SB) vaccine had levels of antibodies to the Hib polysaccharide polyribosylribitol phosphate (PRP) greater than 0.15 microgram ml-1 as did 97% of those receiving Hib (PM) vaccine 1 month after administration of the final vaccine dose. Subjects in all three groups demonstrated an immunological response to pertussis, diphtheria and tetanus antigens. The geometric mean titres of the group given Hib (SB) and DTPw vaccine mixed in the same syringe were lower than the other groups. There were no apparent differences between the treatment groups in the incidence of local or systemic reactions, or serious adverse events. This study has confirmed that it is possible to halve the number of injections necessary to offer protection, with advantages to parents, children, doctors and nurses, using a combined DTPwHib vaccine and in accordance with the UK's accelerated primary immunisation schedule at two, three and four months of age.