The consensus view about the constitution of the T cell receptor repertoire has shifted greatly even during this decade. Although the discovery of autoimmunity in the fifties had clearly shown that a repertoire must exist directed against self antigens, the extent of this repertoire was not fully appreciated. In our work we have tried to elucidate the nature of the antigenic specificities against which this self-directed repertoire is directed. The non-tolerized (residual) self-directed repertoire is a direct consequence of the hierarchy of antigenic determinant display, and is the most important influence in the organism s choice of which T cells to delete. Certain determinants remain "silent" and are neither displayed in the thymus nor in the periphery: these are a heterogeneous group which are invisible to T cells for a variety of reasons. One reason relates to the processing and presentation of determinants, and a second derives from the nature of the T cell receptor (TcR) and the avidity of the T cell for its target specificity.