Studies of TCR expression in antigen-specific immune responses have provided a large body of information correlating the primary structure of the TCR with specificity. Much has been learned regarding the mechanisms for the generation of TCR diversity and the selection of TCR V elements in immune responses. However, the exact nature of the trimolecular complex found during the TCR/antigen/MHC interaction awaits the structural characterization of the TCR by crystallography. Just as describing the three-dimensional structure of MHC molecules and defining their putative antigen-binding site have made it possible to interpret with fresh insight the wealth of functional data regarding antigen/MHC binding and alloreactivity, similar characterization of the TCR structure will allow more definitive interpretation of all the current data with respect to TCR usage in antigen-specific immune responses. It should also facilitate the study of the molecular basis of positive and negative TCR selection during thymic ontogeny. Finally, the identification of T-cell superantigens and their ability to stimulate T cells on the basis of TCR V beta expression alone has defined a new and distinct type of TCR/ligand interaction. The fact that bacterial products such as staphylococcal and streptococcal toxins as well as the mitogenic moiety produced by some mycoplasma are included within this class of antigens suggests that TCR recognition of superantigens may have major clinical significance. It is important to determine the structural basis of T-cell activation by these antigens.