Most mammalian tissues contain a single proteasome species: constitutive proteasomes. Tissues able to express, next to the constitutive proteasome catalytic activities (beta 1c, beta 2c, beta 5c, the three homologous activities, beta 1i, beta 2i and, beta 5i, may contain numerous distinct proteasome particles: immunoproteasomes (composed of beta 1i, beta 2i and beta 5i) and mixed proteasomes containing a mix of these activities. This work describes the development of new subunit -selective activity -based probes and their use in an activity -based protein profiling assay that allows the detection of various proteasome particles. Tissue extracts are treated with subunit -specific probes bearing distinct fluorophores and subunit -specific inhibitors. The samples are resolved by native polyacrylamide gel electrophoresis, after which fluorescence -resonance energy transfer (FRET) reports on the nature of proteasomes present.