Antimyosin monoclonal antibody is a specific marker of impaired sarcolemmal integrity resulting not only from ischemia but also from non-ischemic myocardial injury, such as infection, inflammatory, or immunologic reactions, and alcohol or drug intoxication. In addition, antimyosin accumulation has been demonstrated in some forms of cardiomyopathy with unknown as well as known etiologies. Antimyosin positivity appears to indicate precisely the extent of myocardial necrosis and to reflect cardiac dysfunction in an acute stage of active myocardial damage caused by ischemia and inflammation. However, the correlation is not necessarily good in the chronic stages of the disease or in idiopathic cardiomyopathic hearts; in other words, cardiac antimyosin uptake can be detected even in myocardial tissue with a normal histologic appearance independent of the presence of inflammatory responses, myocyte necrosis, or functional or morphologic deterioration. Thus, antimyosin is useful not only for detecting and quantifying acute myocardial necrosis but also for specifically identifying ongoing or latently progressing myocardial degeneration and sarcolemmal disruption, which will certainly lead to myocardial necrosis. These characteristics may contribute to the early detection of myocardial damage and lead to the investigation of pathophysiologic mechanisms. Further progress in immunologic and radiolabeling techniques is necessary for better specificity and less antigenicity against humans in antimyosin monoclonal antibody imaging. (Jpn Circ J 1998; 62: 77 - 82)