The Proteinase-3 (PR-3) antigen is 29 kD protein found in the primary granules of neutrophils and monocytes. It is a serine protease with broad proteolytic activity against a variety of extracellular matrix proteins (1). ImmunoVision’s PR-3 is purified fom a human promyelocytic cell line using detergent extraction, salt fractionation, and chromaographic techniques. Anti-neutrophil cytoplasmic antibodies (ANCA) are autoantibodies against various lysosomal enzymes (2). Staining by indirect immunofluorescence (IFA) shows two main staining patterns: cytoplasmic (c-ANCA) and perinuclear (p-ANCA) (2,3). The c-ANCA pattern is caused by autoantibodies to PR-3 (4,5). Anti-PR3 autoantibodies are found in 84-100% of patients with Wegener’s granulomatosis (3,6). In addition, PR-3 antibody titer may correlate with disease activity (2).