Haemophilus capsular polysaccharide-tetanus toxoid conjugate (PRP-T) and diphtheria-tetanus-pertussis (DTP) vaccines were administered in a single syringe (group 1) or separate syringes (group 2) to 284 infants at 2, 4, and 6 months of age. Group 1 infants had a slightly greater incidence of local reactions. Systemic reactions were similar. The geometric mean titers of polyribosylribitol phosphate (PRP) serum antibody concentrations after the third dose of PRP-T vaccine were 4.8 and 4.3 micrograms/ml for groups 1 and 2, respectively. Antibody responses to DTP antigens were also similar. The immunogenicity and safety of the PRP-T and DTP vaccines are equivalent when the vaccines are administered in separate syringes or the same syringe to infants.