Combined vaccines consisting of diphtheria (D), tetanus (T), whole cell pertussis (Pw), and Haemophilus influenzae type b (Hib) conjugates are safe and immunogenic; however, there has been a tendency toward lower antibody responses in persons receiving mixtures of DTPw and Hib vaccines. Possible explanations are physical or chemical incompatibilities or immunologic interference. Mixtures of DT-acellular pertussis (Pa) and Hib conjugates have been evaluated as booster vaccinations in children, and preliminary data indicate good safety and acceptable immunogenicity. A study evaluating the combined administration of DTPa, Hib conjugate, and inactivated polio vaccine in infants is ongoing in Finland. Preliminary data suggest that the combination is safe. When the first serum samples were assayed, no major differences in pertussis, diphtheria, and tetanus antibodies were found among children given the vaccines mixed or separately. Hib and polio antibody results are not yet available.