(R)-MG132 is a potent, cell permeable and reversible inhibitor of proteasomes. It can inhibit proteasome activity in lysates of J558L multiple myeloma cells and EMT6 breast cancer cells. In comparison to (S)-MG132, the (R)-MG132 stereoisomer is a more effective inhibitor of chymotrypsin-like (ChTL), trypsin-like (TL), and peptidylglutamyl peptide hydrolyzing proteasome (PGPH) activities.