Myeloperoxidase (MPO) is an enzyme released by leukocytes and some macrophages and elevated levels indicate an ongoing inflammatory process. MPO is also involved in the degradation of the plaque matrix in atherosclerosis. Increased serum concentrations of MPO would indicate both an inflammatory process and plaque instability. An immunoassay for MPO has been approved for use in high risk patients with ACS. More study is required to learn if the MPO levels provide additional information than troponin levels in risk stratification.