Proteasomes catalyze the non‐lysosomal, ATP‐dependent selective breakdown of ubiquitinated proteins and are thought to be responsible for MHC class I‐restricted antigen presentation. Recently, we reported that gamma interferon (IFN‐γ) induced not only marked synthesis of the MHC‐encoded proteasome subunits LMP2 and LMP7, but also almost complete loss of two unidentified proteasome subunits tentatively designated as X and Y in various human cells. Here, we show that subunit X is a new proteasomal subunit highly homologous to LMP7, and that subunit Y is identical to the LMP2‐related proteasomal subunit delta. Thus, IFN‐γ appears to induce subunit replacements of X and Y by LMP7 and LMP2, respectively, producing 'immuno‐proteasomes' with the functional diversity responsible for processing of endogenous antigens.