Microparticles are small membrane-bound vesicles released from activated and dying cells. As shown in a study of primary Sjogren's syndrome, systemic lupus erythematosus and rheumatoid arthritis, levels of microparticles in the blood, as measured by a solid-phase prothrombinase assay or flow cytometry, are increased with autoimmunity. Among patients with these conditions, however, particle numbers were inversely related to disease activity and levels of the enzyme secretory phospholipase A2 that can digest membrane lipids and perhaps cause particle loss. These findings suggest microparticles as novel biomarkers for autoimmunity, with levels reflecting events leading to their loss as well as production.