The impact of the conformation, lipooligosaccharide (LOS)-depletion and the presentation form of outer membrane protein PorA from Neisseria meningitidis (PorA) subtype P1.7-2,4 on the immune response in mice was studied. Native PorA was purified from outer membrane vesicles (OMVs) derived from meningococci and reconstituted into liposomes. The conformation of PorA after purification from OMVs and reconstitution in liposomes was monitored by use of electrophoretic and spectroscopic techniques and compared with the conformation of PorA in outer membrane complexes (OMCs) and heat-denatured PorA. The antigenicity of the PorA formulations was measured by ELISA by using a bactericidal anti-P1.4 monoclonal antibody. Immunogenicity was determined in Balb/c mice. PorA-specific IgG, isotype distribution and bactericidal activity were measured after subcutaneous immunization. In all formulations except in heat-denatured OMVs, PorA was present as trimers. The lipooligosaccharide (LOS) content was reduced by 96% in the purified protein with respect to the original OMVs. The antigenicity of purified PorA (i.e. ELISA response) was substantially higher as compared to PorA in liposomes, OMVs or OMCs. The results of the immunogenicity studies showed that all formulations were able to induce comparable IgG titers. However, whereas the antibodies raised by OMVs were bactericidal, the antibodies elicited by immunization with purified PorA were unable to kill meningococci. Remarkably, the ability to induce bactericidal antibodies was fully recovered by incorporation of the purified PorA into liposomes, in the absence of other adjuvants, as compared to LOS-containing OMVs.