Antigen presentation by activated human and rat CD4⁺ T cells has long been known to induce hyporesponsiveness due to a combination of anergy and apoptosis. It has been assumed that no such phenomenon occurs in mice due to the inability of mouse T cells to synthesize major histocompatibility complex (MHC) class II molecules. There have been several recent descriptions of the transfer of molecules, including MHC molecules, from antigen-presenting cells (APCs) to T cells. Here, we describe the acquisition of MHC class II molecules by T-cell receptor (TCR)–transgenic T cells and T-hybridoma cells following culture with APCs. Acquisition was markedly enhanced by T-cell activation either due to cognate recognition of antigen or anti-CD3 activation. When activation was induced by antigen recognition, preferential acquisition of complexes of class II molecules displaying cognate peptide was observed; in contrast, following activation by anti-CD3 the acquisition of class II molecules was MHC unrestricted. T cells that had acquired MHC class II:peptide complexes were able to act as APCs and induced proliferation and interleukin-2 secretion by resting T cells. However, when activated T cells that had acquired MHC class II:peptide complexes engaged in T:T interactions, this led to an increase in apoptosis and the induction of hyporesponsiveness. These results raise the possibility that the acquisition of MHC class II:peptide complexes by T cells during an immune response may serve to limit clonal expansion, including that induced by alloantigen following tissue or stem cell transplantation.