Experimental autoimmune myocarditis (EAM) resembles the giant cell myocarditis seen in humans, and recurrent forms lead to dilated cardiomyopathy. EAM has been shown to be a T cell-mediated autoimmune myocarditis. We have previously shown that cDNA encoding Vbeta complementarity-determining region (CDR) 3 from heart- and pericardial space-infiltrating T cells in EAM induced by rod cardiac myosin contains more restricted sequences than that from normal spleen T cells. Recently, it has become apparent that several epitopes of EAM exist in rod cardiac myosin; therefore, T cells infiltrating into lesions may recognize certain epitopes in EAM induced by rod cardiac myosin. In this study, we examined heart- and pericardial space-infiltrating T-cell clonotypes in EAM induced by synthetic peptides of cardiac myosin. EAM was produced by immunization with synthetic peptides corresponding to N-terminally acetylated amino acids 1539 to 1555 of rat cardiac myosin alpha heavy chain. Five of 12 rats receiving synthetic peptides developed macroscopic signs of myocarditis. To examine T-cell receptor (TCR) Vbeta expression and CDR3 of the TCR beta chain of lesion-infiltrating T cells in EAM, total RNA was isolated from heart, pericardial effusion, spleen, lymph node, and peripheral blood. TCR Vbeta expression of the T cells infiltrating the lesions revealed a predominance of Vbeta4. On the basis of single-strand conformation polymorphism analysis for CDR3 of the TCR Vbeta4 chain, heart- and pericardial space-infiltrating T cells were considered to be oligoclonal, whereas spleen, lymph node, and peripheral blood in a rat with EAM and spleen in a native rat were considered to be polyclonal. In the same rat, clonotypes of heart-infiltrating T cells were almost the same as those of pericardial space-infiltrating T cells. Furthermore, on sequence analysis for CDR3 of the TCR Vbeta4 chain, the amino acid motifs were similar among T cells infiltrating into lesions of different EAM rats. In the present study, TCR beta chains of heart- and pericardial space-infiltrating T cells in EAM induced by synthesized peptide consisting of 17 amino acids were examined. Vbeta4+ T cells with similar Vbeta CDR3 motifs that infiltrate the heart and pericardial space may recognize the same epitope.