Dihydroeponemycin, an analogue of the antitumor and antiangiogenic natural product eponemycin, selectively targets the 20S proteasome. Dihydroeponemycin covalently modifies a subset of catalytic proteasomal subunits, binding preferentially to the IFN-gamma-inducible subunits LMP2 and LMP7. Dihydroeponemycin-mediated proteasome inhibition induces a spindle-like cellular morphological change and apoptosis.