The ability of activated T cells to present foreign antigens through the MHC class II pathway has been shown in the case of human, rat and mouse T cells. In the present study, the ability of activated T cells to present their endogenous TCR in association with MHC class II molecules to CD4+ T cells was shown. Upon activation mouse T cells downregulate their surface TCR, which are degraded into peptides in endosomal/lysosomal compartments. The idiopeptides (peptides derived from the variable region of the TCR) are presented to cognate anti-idiotypic CD4+ T cells, resulting in activation and proliferation of these cells. Interaction of idiotypic and anti-idiotypic T cells brought about by presentation of TCR idiopeptide may have important implications for T-cell vaccination and perpetuation of T-cell memory not requiring persisting antigen or long-lived memory cells.