MHC class I molecules present peptides generated by processing of endogenously synthesized proteins to CD8+ T lymphocytes. Recently, large proteolytic complexes, termed proteasomes, were implicated in antigen processing. Two proteasomal subunits, LMP2 and LMP7, are encoded within the MHC class II region, but their precise role in antigen processing is unknown. We have generated mice that harbor a disruption in their LMP2 gene. Proteasomes purified from spleen and liver of these mutant mice exhibit altered peptidase activities, and antigen-presenting cells showed reduced capacity to stimulate a T cell hybridoma specific for H-2Db plus a nucleoprotein epitope of an influenza A virus. The mutant mice have reduced (6005 70% of wild type) levels of CD8+ T lymphocytes and generate 5 to g-fold fewer influenza nucleoproteinspecific cytotoxic T lymphocyte precursors. These findings indicate that LMP2 influences antigen processing.