Clonal T-cell anergy has been proposed as a mechanism to ensure peripheral tolerance in vivo. Anergy has been reported to result from T cell activation with inappropriate antigen-presenting cells (APC) or, in the case of CD4+ T cells, also by altered peptide ligands. This study reveals that altered hapten ligands can also induce anergy in CD8+ T cells. The Kb-restricted, trinitrophenyl (TNP) specific cytotoxic T lymphocyte (CTL) clone E6 was found to lyse target cells presenting the TNP-modified peptides M4L-TNP (derived from mouse serum albumin) or O4TNP (derived from chicken ovalbumin), but not the corresponding dinitrophenol (DNP)-modified peptides. However, whereas M4L-DNP was found totally unreactive, O4DNP antagonistically inhibited M4L-TNP-mediated kill if expressed on the same target cell. Moreover, when presented alone on APC, O4DNP, but not M4L-DNP, induced anergy in clone E6 by preventing its subsequent proliferative response to M4L-TNP. The anergic state did not affect agonist-specific cytolysis or T-cell receptor (TCR) down-modulation by the anergized CTL, and proliferative responses were regained upon addition of interleukin (IL)-2 or IL-12 plus IL-18. These findings substantiate the similarity between hapten-and peptide-recognition by T cells. The induction as well as the reversal of anergy in CD8+ CTL may thus be of relevance not only in autoimmunity or tumour rejection, but also in contact hypersensitivity reactions to haptens.