Proteasomes are the major source for the generation of peptides bound by MHC class I molecules. To study the functional relevance of the IFN-γ-inducible proteasome subunits low molecular mass protein 2 (LMP2), LMP7, and mouse embryonal cell (MEC) ligand 1 in Ag processing and concomitantly that of immunoproteasomes, we established the tetracycline-regulated mouse cell line MEC217, allowing the titrable formation of immunoproteasomes. Infection of MEC217 cells with Adenovirus type 5 (Ad5) and analysis of Ag presentation with Ad5-specific CTL showed that cells containing immunoproteasomes processed the viral early 1B protein (E1B)-derived epitope E1B 192–200 with increased efficiency, thus allowing a faster detection of viral entry in induced cells. Importantly, optimal CTL activation was already achieved at submaximal immunosubunit expression. In contrast, digestion of E1B-polypeptide with purified proteasomes in vitro yielded E1B 192–200 at quantities that were proportional to the relative contents of immunosubunits. Our data provide evidence that the IFN-γ-inducible proteasome subunits, when present at relatively low levels as at initial stages of infection, already increase the efficiency of antigenic peptide generation and thereby enhance MHC class I Ag processing in infected cells.