Immunoproteasomes containing the IFN-inducible subunits β1i (LMP2), β2i (MECL-1) and β5i (LMP7) alter proteasomal cleavage preference and optimize the generation of peptide ligands of MHC class I molecules. Here, we report on an unexpected new function of immunoproteasome subunits for the survival and expansion of CD4(+) and CD8(+) T cells during viral infection of mice. The effect of immunoproteasome subunit deficiency on T-cell survival upon adoptive transfer was most prominent for the lack of LMP7 followed by MECL-1 and LMP2. The survival of T cells in uninfected mice or the homeostatic expansion after transfer into RAG-2(-/-) mice was not affected by the lack of the immunosubunits. Lymphocytic choriomeningitis virus (LCMV)-specific CD8(+) T cells lacking LMP7 or MECL-1 started to divide after transfer into LCMV-infected mice but experienced a considerable cell loss within 2 days after transfer. We provide strong evidence that the loss of immunoproteasome-deficient T cells after transfer is not a consequence of graft rejection by the host, but instead is based on the requirement for immunoproteasomes for the survival of T cells in LCMV-infected mice. Therefore, the immunoproteasome may qualify as a potential new target for the suppression of undesired proinflammatory T-cell responses.