Immunogenicity and vaccine potency of carrier proteins of two different PRP-tetanus toxoid (PRP-T) conjugated vaccines, produced using different size PRP (Act-Hib & Amvax Hib-T), and one PRP-CRM197 (Hib-TITER) were studied. The immunogenicity and the vaccine potency of the carrier component of the tested PRP-conjugated vaccines, and their influences on the potency of the tetanus toxoid (T) and of the diphtheria toxoid (D) component of diphtheria toxoid-tetanus toxoid-acellular pertussis-inactivated polio vaccine (DTaP-IPV) were variable. The T component of Act-Hib (large size PRP) was as immunogenic and potent as the T component of the DTaP-IPV vaccine, and a combination of Act-Hib and DTaP-IPV resulted in a more than five-fold increase in the potency of the T However, Amvax Hib-T (small size PRP) did not show any anti-T response on its own, and a combination of Amvax Hib-T and DTaP-IPV did not affect the T potency of the DTaP-IPV vaccine. In immunogenicity studies with multiple shots, Hib-TITER (small size PRP) produced significantly less anti-D antibodies than non-conjugated D. Hib-TITER did not show any D potency on its own, while a combination of a Hib-TITER and DTaP-IPV increased the potency of the D component of DTaP-IPV vaccine significantly. Thus, in the case of a combination of T-and D-containing vaccines with a PRP-conjugated vaccine in which either T or CRMI 97 has been used as the carrier, the influence of these carriers on basic immunogenicity and vaccine potency of T and D, respectively, should be considered carefully. We propose the techniques employed in this study for the quality control of combined vaccines consisting of diphtheria, tetanus, and PRP-conjugated vaccines.