Proteinase 3 (PR3), a neutrophil granule serine protease, is the major autoantigen for autoantibodies in the systemic vasculitic disease, Wegener's granulomatosis. It is also found to be involved in various inflammatory diseases including Crohn's disease, rheumatoid arthritis, cystic fibrosis, and gingivitis. However, there is no high quality antibody available to detect endogenous PR3 in biological samples such as plasma and tissue. Several commercial anti-PR3 monoclonal antibodies (MAbs) were obtained by using HMC-1/PR3 cell granule extracts as the antigen, but the resulting antibodies could not be applied for immunoblotting or other immunological methods. Therefore, we produced human recombinant PR3 in Escherichia coli and developed several MAbs that are highly sensitive and can be used for immunoblotting, FACS analysis, and immunofluorescent staining. The PR3 MAbs recognized both rhPR3 and human plasma-derived neutrophil PR3 in reducing and non-reducing conditions at low nanogram levels. In addition, new MAbs detect endogenous PR3 from normal human plasma and urine with high specificity. The new anti-PR3 MAbs will be an essential tool for investigating the role of PR3 in inflammatory and autoimmune diseases.