The safety and immunogenicity ofHaemophilus influenzaevaccine (tetanus toxoid conjugate (PRP-T)) administered concurrently in separate sites or mixed in the same syringe with diphtheria and tetanus toxoids, pertussis vaccine and inactivated poliomyelitis vaccine were assessed in 439 infatns at 2, 4 and 6 months of age. The proportions with local redness, tenderness and swelling in the separate and combined groups were 18%vs.11% (P< 0.001), 27%vs.24% and 15%vs.13%, respectively. Systemic reactions occurred at similar rates in both groups. The combined vaccine induced tetanus and diphtheria antitoxin titers ≥ 0.01 IU/ml in 99.5 and 99.1% of infants, pertussis agglutinin titers ≥ 64 in 92.4%, anti-polyribosylribitol phosphate titers ≥ 0.15 μg/ml in 93.8% and ≥ 1.0 μg/ml in 75% and polio-neutralizing titers ≥ 8 in > 98 ≥ of infants. However, antibody concentrations to PRP-T. some pertussis antigens and tetanus toxoid were significantly lower after combined than after separate injections of DPT/diphtheria and tetanus toxoids, pertussis vaccine and in-activated poliomyelitis vaccine and PRP-T. The clinical significance of these differences is not knon, but the interactions observed among the components of the pentavalent vaccine may be of concern because they might influence antibody persistence until the fourth dose is administered.