We studied the reactivity of cardiac graft-infiltrating cells, cultured from endomyocardial biopsy specimens, toward heart endothelial cells (Hec). In two cases, Hec derived from the specific donor heart or Hec compatible with the donor were lysed, but not the syngeneic B cell line. A vessel-derived endothelial cell line was not lysed. Panel studies suggest that the epitopes recognized are, in one case, a Hec-specific peptide presented in the context of HLA-Bw41 and, in the other case, a Hec-specific peptide presented by a subtype of HLA-B44. In conclusion, we showed that cardiac graft-infiltrating cells cultured from endomyocardial biopsy specimens can exhibit cytotoxic reactivity specifically directed against HLA-peptide complexes on Hec.