The synthesis and biological evaluation of ten Michael acceptors containing potential proteasome inhibitors are described. Cellular targets of azide containing inhibitors and were assessed in HEK293T and RAW264.7 cells by a two step labeling strategy, followed by biotin-pulldown, affinity purification, on-bead tryptic digestion and LC-MS(2) identification. This strategy appears to be an attractive alternative to gel-based competition assays.