Alloreactive T lymphocytes play an important role in graft rejection. In the present study, we have analyzed the cytolytic capacity against donor cells of graft infiltrating T lymphocyte cell lines, which were propagated from various endomyocardial biopsies taken at different time points after transplantation, including during a rejection crisis. Also, T cell clones were generated from the rejection biopsy and evaluated for their cytolytic capacity and nucleotide composition of the TCR alpha and beta chains. The results of these studies revealed a strong cytolytic activity against donor cells by T cells derived from the rejection biopsy, whereas from the other biopsies, no cytolytic T cell clones could be established. The T cells that were responsible for this activity, as detected by T cell cloning and TCR gene analysis, could not been identified in earlier biopsies, indicating that these cytolytic cells were recently recruited toward the endomyocardium.