TwoHaemophilus influenzaetype b (Hib) polysaccharide-protein conjugate vaccines were evaluated for immunogenicity in eliciting anti-polyribosyl ribitol phosphate (PRP) antibodies in infant rhesus monkeys. Animals received intramuscular injections of either Hib polysaccharide (PRP)-meningococcal outer membrane protein complex or Hib oliosaccharide-CRM197(HbOC) conjugate vaccines on d 0,28, and 56. Because HbOC contains the CRM197mutant diphtheria toxin fromCorynbacterium diphtheriaeas its protein carrier, the effect of simultaneous injection of diphtheria toxoid on the immunogenicity of HbOC also was evaluated by dividing monkeys vaccinated with HbOC into three groups: HbOC/saline, HbOC/diph-theria and tetanus toxoids, and HbOC/tetanus toxoid (coadministration of HbOC and other vaccine or placebo injected into the flank muscle of different legs). Infant monkeys vaccinated with the PRP-outer membrane protein complex conjugate responded with anti-PRP antibody after the first dose and showed booster responses after the second and third injections. In contrast, infant monkeys vaccinated with HbOC did not respond after three doses of HbOC/saline or HbOC/tetanus toxoid. However, two of three monkeys given concurrent injections of HbOC and diphtheria and tetanus toxoids did respond. The nonresponder monkey to three doses of HbOC and diphtheria and tetanus toxoids did respond to a subsequent injection with PRP-outer membrane protein complex. Thus, concomitant administration of diphtheria toxoid, a common vaccine for human infants, is necessary to elicit an anti-PRP antibody response to HbOC. This finding suggests that the CRM197protein, at the 12.5-μg dose of protein in the HbOC used in this study, may not be sufficiently immunogenic in naive infant rhesus monkeys to permit an antibody response to the Hib oligosaccharide component of the conjugate.