Abstract: The present invention relates to methods for diagnosing acute myocardial infarction through the measurement of the level of marinobufagin-like immunoreactivity in the blood of patients suspected of this diagnosis; a method for treating patients with acute myocardial infarction with antibody to marinobufagin, a bufodienolide, to prevent the occurrence of cardiac arrhythmias; antibodies which specifically recognize marinobufagin or other bufodienolides; hybridomas producing these antibodies; a process for preparing such antibodies; and an immunoassay method for marinobufagin for research purposes using its specific antibody.The antibodies of the present invention make it possible to conveniently measure bufodienolides with specificity and high sensitivity. This is useful in determining the existence and degree of hypertension and myocardial infarction, and in treating myocardial infarction.