Matching for human leucocyte antigens (HLA) is important for graft survival in kidney transplantation. Nevertheless, most patients receive a kidney graft with multiple HLA mismatches. Some of these mismatches seem to be more harmful than others. By studying the effect of single HLA mismatches in the context of the patients' own HLA, we have previously identified donor/recipient combinations with a significantly higher incidence of early graft failure, the so-called taboo combinations. In the present study we investigated whether a higher cytotoxic T lymphocyte (CTL) response towards taboo mismatches may be involved in this phenomenon. CTL reactivity was determined both in taboo and control combinations by in vitro CTL precursor assays, using peripheral blood mononuclear cells and proximal tubular epithelial cells as target cells. Inhibition studies with CD8-antibody as well as Cyclosporin A were performed to identify high avidity and primed CTLs. Furthermore, in committed CTLp assays indirect recognition of the taboo mismatch was tested using synthetic peptides. The CTL precursor frequencies in taboo combinations were always lower than the CTL precursor frequencies in control combinations. No difference in avidity and activation status of the CTLs could be detected when taboo combinations were compared with the controls. In the committed CTLp assays no reactivity towards any of the synthetic peptides was observed. The significantly poorer graft survival of taboo combinations cannot be explained by a higher number of donor-specific CTLs. Furthermore, the avidity or activation status of these CTLs does not provide a clue to the taboo phenomenon.