Immunoassays for cardiac myosin light chains (LC) are important biochemical tests for diagnosis of acute myocardial infarction (AMI). LC appears in the serum 4-12 hours after AMI. The most unique characteristic of the time-course of LC is that the elevation of LC in the serum lasts for 1-2 weeks, which allows the retrospective diagnosis of AMI when cardiac enzymes in the serum are decreased to the normal level. This long time-course is due to the continuous liberation of LC from the infarcted myocardium, which could be confirmed by the cardiac scintigram using anti-myosin heavy chain antibody. The serum LC level is also useful in predicting the extent of the cardiac wall damage since the peak LC level reflected well the ventricular ejection fraction or the presence of mechanical complications, such as the formation of ventricular aneurysm. Immunoassays for cardiac LC have recently been improved by the introduction of ELISA using two monoclonal anti-light chain antibodies. ELISA can be finished within 3 hours, which will help to make cardiac LC measurement one of the most important biochemical tests for diagnosis of acute myocardial infarction.