T cells bearing alphabeta T cell receptors (TCRs) recognize antigens in the form of peptides bound to class I or class II major histocompatibility proteins (MHC). TCRs on mature T cells are usually very specific for both peptide and MHC class and allele. They are picked out from a precursor population in the thymus by MHC-driven positive and negative selection. Here we show that the pool of T cells initially positively selected in the thymus contains many T cells that are very crossreactive for peptide and MHC and that subsequent negative selection establishes the MHC-restriction and peptide specificity of peripheral T cells. Our results also suggest that germline-encoded TCR variable elements have an inherent predisposition to react with features shared by all MHC proteins.