Immunoglobulin A1 (IgA1) proteases from Haemophilus influenzae are extracellular proteases that specifically cleave the hinge region of human IgA1, the predominant class of immunoglobulin present on mucosal membranes. The IgA1 proteases may have the potential to cleave IgA1 complexes in the kidney and be a therapeutic agent for IgA1 nephropathy (IgAN), a disease characterized by deposition of the IgA1 antibody in the glomerulus. We have screened for the expression of recombinant H. influenzae IgA1 protease by combining various expression plasmids, IgA1 protease constructs, and E. coli strains under multiple conditions. Using the method we have developed, approximately 20-40 mg/L of soluble and active H. influenzae IgA1 protease can be produced from E. coli strain C41(DE3), a significant increase in yield compared to the yield upon expression in H. influenzae or other related bacteria.