Myoglobin is a cytoplasmic, soluble, heme protein present in muscle cells having a molecular weight of approximately 17,000 Daltons. Because of its relatively small size, high cellular concentration, and cytoplasmic location, myoglobin is released earlier than other cardiac markers following cellular necrosis or injury. Blood concentrations of myoglobin increase above the reference range within the first two hours following injury, reaching a peak between six and eight hours after the onset of symptoms. Myoglobin returns to baseline or normal concentrations within 20-36 hours after tissue damage. Myoglobin is present in all types of muscle cells. Therefore, its appearance in blood is not necessarily associated with myocardial injury. Blood myoglobin concentrations may be elevated as a result of a variety of conditions that produce muscle damage. These include trauma, ischemia, surgery, exercise and a variety of degenerative muscular diseases.