Recognition of class I MHC antigens involves interaction between TCRs of cytotoxic T lymphocytes (CTL) and the two a helices of MHC molecules. Using a combined panel of H-2K b mutants selected by either a CTL clone or MAbs, we have shown evidence that the TCRs of 59 K b -specific CTL clones shared a common binding pattern on the H-2K b molecule. Mutations of amino acid residues at the C-terminal regions, but not the N-terminal regions, of the a helices abrogated the recognition by the majority of the clones. The data suggests that TCRs predominantly recognize the class I MHC molecule with an orientation that is parallel to the β-pleated strands and diagonal to the α helices.