The myeloperoxidase (MPO) antigen is a heme-containing protein found in the primary granules of neutrophils and monocytes. It’s primary function is to destroy phagocytosed microorganisms by generating highly reactive oxidative species within the phagosome (1). MPO is a heterodimer consisting of two 59 kD heavy chains and two 14 kD light chains. ImmunoVision’s MPO is purified from a human promyelocytic cell line using detergent extraction, salt fractionation, and chromatographic techniques. Anti-neutrophil cytoplasmic antibodies (ANCA) are autoantibodies against cytoplasmic constituents of neutrophils and monocytes (2). Staining by indirect immunofluorescence (IFA) shows two main staining patterns: cytoplasmic (c-ANCA) and perinuclear (p-ANCA) (3,4). Anti-MPO antibodies usually generate a p-ANCA pattern but not all p-ANCA patterns are caused by anti-MPO antibodies. Anti-MPO autoantibodies are strongly associated with idiopathic microscopic polyangiitis, crescentic glomeronephritis, polyarteritis nodosa, and Churg-Strauss syndrome (5,6). In addition, MPO antibody titer may correlate with disease activity (3).