A smooth muscle layer exists in the parietal endocardium. Notwithstanding previous pioneer work, little has been known on the phenotypes of these smooth muscles. In humans as in animals, smooth muscles show two distinct phenotypes, the 'synthetic' and 'contractile' states, which can be differentiated on the basis of the expression of two types of myosin heavy chain isoforms, SM1 and SM2. In this study, using SM1- and SM2-specific monoclonal antibodies, we performed immunohistochemical examinations to determine the phenotype of the endocardial smooth muscles. We report here that the endocardial smooth muscles have the contractile phenotype.