The proteasome is a non-lysosome protein degradation system that requires metabolic energy and polymerization of ubiquitin. The 26S proteasome is made up of two subcomplexes (a 20S proteasome and one or two19S regulatory particles). Rapid and gentle isolation of proteasome from cells/tissues is essential for the studies of proteasome structure and function. The proteasomes have been found mainly localized in cytosol but also detected in nucleus. The nuclear proteasomes are excluded with this kit (Nuclear Proteasome Enrichment Kit is available under Cat # PN-041). The protocol is simple and rapid with high yield. The gentle protocol maintains the association of proteasomes with ubiquitin and other proteins. This kit is useful for the studies of proteasome structure and function. The enriched proteasomes can also be used as a starting material for affinity purification.