PROTEASOMES are highly conserved macromolecular structures which function as endopeptidases^1–3. They are found in the cyto-plasm and nucleus of eukaryotic tissues and consist of at least 14 non-identical subunits with molecular masses ranging from ∼20 to 32K. Proteasomes are essential in the selective degradation of ubiquitinated and certain non-ubiquitinated proteins, acting as the proteolytic core of an energy-dependent 26S (1,500K) proteolytic complex. Two proteasome subunits, LMP2 and LMP7 (refs 4–7), are encoded within the major histocompatibility complex (MHC), implicating proteasomes in antigen processing^8,9. Here we deter-mine the function of these two MHC-linked subunits by comparing the proteolytic activities of purified proteasomes containing (LMP⁺) or lacking (LMP⁻) these components. We find that pro-teasomes of both types have endopeptidase activity against sub-strates bearing hydrophobic, basic or acidic residues immediately preceding the cleavage site (the PI position) and at sites following asparagine, glycine and proline residues. The activity of LMP⁺ proteasomes is much higher than that of LMP⁻ proteasomes against substrates with hydrophobic, basic or asparagine residues at PI, whereas their activities are comparable when acidic and glycine residues are present at PI. The MHC-linked LMP2 and LMP7 subunits therefore function to amplify specific endopeptid-ase activities of the proteasome.