The specificity of mature CD8+ and CD4+ T lymphocytes is controlled by major histocompatibility complex (MHC) class I and class II molecules, respectively. The MHC class specificity of T cells is stringent in many assays, but is less evident when cells are supplemented with exogenous lymphokines. The repertoire of T cells is shaped through contact with MHC molecules in the thymus and involves a complex process of positive selection and negative selection (tolerance). Tolerance of immature T cells to MHC molecules can reflect either clonal deletion or anergy and results from intrathymic contact with several cell types, including epithelial cells and cells with antigen-presenting function. Unlike immature T cells, mature T cells are relatively resistant to tolerance induction. In certain situations partial unresponsiveness of mature T cells can be achieved by exposing T cells to foreign MHC molecules expressed on atypical antigen-presenting cells. Tolerance is rarely complete, however, and the precise requirements for tolerizing mature T cells are still unclear.