Gonococcal virulence is thought to rely on multiple characteristics including the production of an extracellular protease specific for human IgA1. Using a sensitive filter assay we have isolated an Escherichia coli clone which harbours the gene of Neisseria gonorrhoeae MS11 IgA protease on a multicopy number plasmid. This clone secrets IgA protease activity to an extent similar to that of the parental MS11 strain. By exonucleolytic digestion of the cloned insert we obtained a fragment of 4.6 kb which could not be shortened further without loss of IgA protease expression. Compared with the cloned IgA protease gene from N. gonorrhoeae F62, this minimal gene segment shows marked differences in the arrangement of restriction sites. We suppose that these differences determine strain-specific variations of N. gonorrhoeae IgA proteases and also affect the secretory properties of the enzyme when produced in E. coli. A novel purification procedure developed for IgA protease of N. gonorrhoeae allowed us to correlate the enzyme activity with a distinct protein band in SDS acrylamide gels. By comparison with the enzyme prepared from the E. coli clone, we identified a 105-kd protein as the extracellular form of gonococcal IgA protease.