Abstract: The invention provides a process for the insulation of an antigenic composition substantially free from cells and lipopolysaccharide, the said composition comprising a complex having a polysaccharide constituent and a protein constituent, wherein the polysaccharide constituent comprises capsular polysaccharide specific to serogroup B of N. meningitidis, and the protein constituent comprises N. meningitidis outer-membrane protein selected from the class consisting of those proteins specific to serotype 6, those proteins specific to serotype 2 and having a single dominant component having a molecular weight of 42,000.+-.3,000, and immunological equivalents of such proteins. Preferably the composition also comprises a pharmacologically acceptable metal either as an adjuvant in admixture with the polysaccharide-protein complex, or in complex therewith to provide a triple metal-polysaccharide-protein complex. The composition is useful for the prophylaxis or treatment of bacterial disease in mammals such as man.