Myeloperoxidase (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. MPO is released in the extra cellular medium where hypochlorous acid leads to chlorination of proteins leading to products as 3-chlorotyrosine. Furthermore it leads to oxidation of low density lipoprotein (LDL) and apolipoprotein A-I, the primary protein of HDL resulting in the disruption of HDL functions as cholesterol efflux. 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 Wegener"s 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.The classical MPO assay is an enzymatic assay for activity of MPO. This classical MPO assay is hampered by the presence of inhibitory compounds in tissue homogenates and plasma. In this type of assays spiking often gives unreliable results. The rat MPO elisa is not influenced by inhibitors of the enzyme activity.