Ultrastructural and immunohistochemical studies of six cardiac myxomas were carried out to further elucidate the histogenesis of this tumor and the cell character of myxoma cell. Ultrastructurally myxoma cells have varying degrees of endothelial characteristics with prominent vasoformative activity. Immunohistochemically, blood group H-antigen and Factor VIII-related antigen were identified in tumor cells of all six cases. Prostaglandin I2 was demonstrated in five. The contractile proteins (actin and myosin), basement membrane components (laminin and fibronectin) were identified in all cases. Intermediate filament vimentin was demonstrated in two cases. On the other hand, desmin, keratin, and glial fibrillary acidic protein were not demonstrated. This result further supports the concept of the endothelial origin of cardiac myxoma rather than origin form multipotential mesenchymal cells. Myxoma cells infiltrated and covered the fibrin thrombi attached to the surface of the tumor, and the fibrin thrombi were replaced by myxoid matrix as the myxoma cells infiltrated it. This finding suggests that fibrin thrombi may be related to the growth of cardiac myxoma and active synthesis of prostaglandin I2 in myxoma cells may be due to intimate contact with platelet-rich fibrin thrombi attached to the tumor.