A method of the isolation and purification of IgA1 protease from a culture of Neisseria meningitidis serogroup A has been developed. Three inactivated intermediates of the production of the meningococcal vaccine, a culture liquid, as well as a supernatant and precipitate obtained by the precipitation of bacterial cells by cetavlon, served as a starting material. The purity of IgA1 protease was determined by SDS-PAGE. An immunoenzyme assay for determining the IgA1 protease activity has been developed. The yield of the enzyme with a specific activity of 0.5 to 4 million units/mg from 103 g of the cetavlon precipitate (40 l of culture liquid) was about 600 μg. It was shown that IgA1 protease isolated from serogroup A meningococcus is capable of protecting experimental animals (mice) infected with meningococcus of serogroup B. © 2010 Pleiades Publishing, Ltd.