Because inactivated poliovirus vaccine (IPV) andHaemophilus influenzaeb vaccine are advised in many programs and may be incorporated further in other programs, we undertook a study to determine whether the administration of a tetravalent preparation of diphtheria-tetanus-pertussis-IPV mixed in one syringe with tetanus-conjugateH. influenzaeb vaccine (DTP-IPV-PRPT) is associated with increased reactogenicity or interference with immunogenicity of individual vaccine components. In a placebo-controlled, double blind study, a total of 161 infants were enrolled (80 DTP-IPV-PRPT and 81 DTP-IPV-placebo). Vaccine was administered at 2, 4 and 6 months of age. Oral polio-virus vaccine was added at 7 months of age and a booster of oral poliovirus vaccine and DTP-IPV was also administered at 12 months of age, according to the policy in Israel. Local and systemic side effects were similar in both groups except for irritability after the second dose and use of acetaminophen which we observed slightly but significantly more often in the DTP-IPV-PRPT recipients. After the third dose the geometrie mean titers of anti-polyribosyl-ribitol phosphate antibodies were 3.7 and 0.05 μg/ml in the PRPT and placebo groups, respectively (P< 0.001). Higher tetanus antitoxin titers were observed among recipients of DPT-IPV-placebo (1.1 IU/mlvs.0.7 IU/ml,P= 0.003). A similar trend was found for pertussis agglutinin titers (93.4vs.65.4,P= 0.054). No difference was observed for anti-diphtheria toxoid and poliovirus 1, 2 and 3. Protective titers against diphtheria and tetanus (≥0.02 IU/ml) after the third dose were demonstrated in ≥99% of all infans. At 18 months of age (6 months after the DTP-IPV booster) no significant difference between the groups was demonstrated showing good priming for all components, although the trend for lower anti-tetanus antibodies was still present. We conclude that although a mild, clinically insignificant interference occurred for some components after the primary series, no interference with priming was shown and that DTP-IPV mixed with PRPT is safe and immunogenic in young infants.