Background: 20S proteasomes, the proteolytic core particles of the major intracellular protein degradative pathway, are potential disease markers because they are detectable in human plasma as circulating proteasomes and their concentrations are increased in patients suffering from various diseases. To investigate the origin of circulating proteasomes, we compared some of their features with those of proteasomes isolated from major blood cells. Methods: We isolated circulating proteasomes from the plasma of 2 patients with rheumatoid arthritis and 2 with systemic lupus erythematosus and from human plasma from healthy donors. We purified the proteasomes to apparent homogeneity and then used electron microscopy for imaging and chromatography for subtype spectrum analysis. We compared subtype results with those from 20S proteasomes purified from 4 major blood cell populations. We also tested proteasomes for enzymatic activity and immunosubunit content. Results: Circulating proteasomes from plasma of healthy donors and from patients with autoimmune disease were found to have the same size and shape as erythrocyte proteasomes, be proteolytically active, and contain standard-and immunosubunits. Chromatography revealed 6 circulating proteasome subtype peaks in healthy donor plasma and 7 in patient donor plasma. Proteasomes from erythrocytes had 3 subtype peaks and