Existing methods of meningococcal OMV preparation involve the use of detergent during disruption of the bacterial membrane. According to the invention, membrane disruption is performed substantially in the absence of detergent. The resulting OMVs retain important bacterial immunogenic components, particularly (i) the protective NspA surface protein, (ii) protein NMB2132 and (iii) protein NMB1870. A typical process involves the following steps: (a) treating bacterial cells in the substantial absence of detergent (b) cezttrifuging the composition from step (a) to separate the outer membrane vesicles from treated cells and cell debris, and collecting the supernatant (c) performing a high speed centrifugation of the supernatant from step (b) and collecting the outer membrane vesicles in a pellet (d) re-dispersing the pellet fi-om step (c) in a buffer (e) performing a second high speed centrifugation in accordance with step (c), collecting the outer membrane vesicles in a pellet (f) re-dispersing the pellet from step (e) in an aqueous medium.
