A vaccine against serogroup B meningococcal disease has been prepared from a B:15:P1.7,16 meningococcal strain (44/76) by fermentor growth and extraction of the bacteria with the detergent deoxycholate. Outer membrane vesicles (OMV) were purified by ultracentrifugation and adsorbed to aluminium hydroxide adjuvant. OMV contained the major class 1, 3, 4 and 5 proteins and some minor high molecular weight protein components. Relative to protein, the vaccine also contained about 8% phospholipid, 7% lipopolysaccharide and 16% deoxycholate. The product was generally non-pyrogenic to humans in ordinary doses and was highly immunogenic in mice and humans. Production and control steps, physical, chemical and immunological data for the vaccine are described.