The monoclonal antibody 2D4 reacts with rat myeloperoxidase (MPO). MPO is a glycoprotein with an alpha2beta2 heteromultimer expressed in all cells of the myeloid linage. MPO is abundantly present in azurophilic granules of polymorphonuclear neutrophils. It is an important enzyme used during phagocytic lysis of engulfed foreign particles which takes part in the defense of the organism through production of hypochlorous acid (HOCl), a potent oxidant. MPO is rapidly released by activated polymorphonuclear neutrophils. Involvement of MPO has been described in numerous diseases such as atherosclerosis, lung cancer, Alzheimer"s disease and multiple sclerosis. Autoimmune antibodies to MPO are involved in Wegeners disease. Since the discovery of MPO deficiency, initially regarded as rare and restricted to patients suffering from severe infections, MPO has attracted more clinical attention. In experimental studies antibodies to MPO can be used for various purposes ranging from flow cytometric analysis to detection of polymorphonuclear neutrophils in tissue sections.