Polysaccharide-protein conjugate vaccines made with different carriers vary in their ability to elicit antipolysaccharide IgG antibody responses in young infants and an adult mouse model, suggesting that the carrier proteins used in the conjugate vaccines differ in their ability to act as carriers, or that additional mechanisms of immunogenicity play a role. A conjugate vaccine of Haemophilus influenzae PRP coupled to the outer membrane protein complex (OMPC) of Neisseria meningitidis serogroup B is immunogenic in children as young as 2 mo of age and is immunogenic in infant rhesus monkeys, an animal model for infant humans. In the present study, PRP-OMPC was found to induce efficient IgM to IgG switching of anti-PRP serum antibody in adult mice, whereas PRP conjugated to two other protein carriers did not. Thus the PRP-OMPC conjugate was examined in order to determine why PRP coupled to OMPC was so immunogenic, even more immunogenic than conjugates made with other carrier proteins. The OMPC carrier differs from the other protein carriers in that the proteins are present in a liposomal form containing lipids (including LPS) derived from the outer membrane of N. meningitidis. We studied the OMPC to see whether the different components or the nature of the OMPC carrier could contribute to its enhanced immunogenicity. Specifically we evaluated the OMPC for both classic Th cell carrier activity and adjuvanticity, and the LPS component of OMPC for systemic polyclonal B cell activation. Carrier recognition of the OMPC moiety of PRP-OMPC was demonstrated. In addition the PRP-OMPC conjugate vaccine was observed to have adjuvant properties for both T cell-dependent and T cell-independent Ag in the absence of LPS-induced systemic polyclonal B cell activation. These observations suggest that in addition to functioning as a classic protein carrier whereby the proteins in OMPC provide Th cell epitopes, the OMPC also has adjuvant activity that distinguishes it from other protein carriers and may contribute to the increased immunogenicity of PRP-OMPC conjugates in animal models.