A rapid, sensitive, and specific marker for the diagnosis of acute myocardial infarction (MI) and the assessment of reperfusion following thrombolytic therapy has been sought by research workers for years. Creatine kinase-MB (CK-MB) is the best biochemical marker currently available to the cardiologist and the emergency room physician for the assessment of patients presenting with symptoms of acute MI. CK-MB is best measured using immunoassay techniques at 3- to 4-hour time intervals during the first 12 hours after onset of symptoms. Other currently available markers include lactate dehydrogenase and its isoenzymes and myoglobin. Future developments include assays for troponin, reported to be a true cardiac-specific marker, and myosin light chains which may have value in noninvasive infarct sizing.