Seven canine donor hearts in which atrial septal defect and tricuspid regurgitation had previously been produced were heterotopically transplanted into the recipients' chest cavities. Indium-111 antimyosin myocardial imaging of the excised heart was performed using a scinticamera. Magnetic resonance imaging was also performed and the T2 relaxation time calculated. Subsequently, these data were correlated with pathological findings, which indicated the degree of rejection. Indium-111 antimyosin uptake was high in moderate and severe rejection, but the T2 relaxation time was prolonged even in mild rejection. Thus indium-111 antimyosin uptake was specific, and the T2 relaxation time was sensitive, for detecting the severity and extent of cardiac rejection. Although ex vivo experimental results have been reported, these new methods allow characterisation and accurate evaluation of myocardial tissue undergoing cardiac rejection.