Abstract : In this final report, we describe our efforts that culminated in the determination of the x-ray crystal structure of a T cell receptor-MHC molecule (TCR/MHC) complex. Based on the structure, we proposed several common characteristics that all TCR/MHC complexes may have. We prepared and analyzed crystals containing mutant peptides that differ from the wildtype peptide in their immunological properties. We have made progress in determining the structure of another T cell receptor bound to the same MHC-peptide complex. When finished, these new structures and their comparison to the first TCR/MHC structure should lead to further insights in how T cell receptors recognize their MHC-peptide ligands. In this final report, we describe our efforts that culminated in the determination of the x-ray crystal structure of a T-cell receptor/MHC molecule (TCR/ MHC) complex. Based on the structure, we proposed several common characteristics that all TCR/MHC complexes may have. We prepared and analyzed crystals containing mutant peptides that differ from the wildtype peptide in their immunological properties. We have made progress in determining the structure of another T-cell receptor bound to the same MHC/peptide complex. When finished, these new structures and their comparison to the first TCR/ MHC structure should lead to further insights in how T-cell receptors recognize their MHC/peptide ligands.