Microparticles (MPs) are small membrane-bound vesicles that arise from activated and dying cells and promote inflammation and thrombosis. To characterize the in vivo release of MPs, we used flow cytometry to measure MPs in the blood of 15 healthy volunteers administered bacterial endotoxin (lipopolysaccharide or LPS) in the presence of a low dose of hydrocortisone with or without inhaled nitric oxide. MPs, defined as particles less than 1.0 μm in size, were assessed following labelling for CD42a, CD14 and CD62E or CD144 antibodies to identify MPs from platelets (PMPs), monocytes (MMPs) and endothelial cells (EMPs). In addition, PMPs and MMPs were labelled with anti-HMGB1 and stained with SYTO13 to assess nuclear acid content. Administration of LPS led to an increase in the numbers of PMPs, MMPs and EMPs as defined by CD62E, as well as the number of MMPs and PMPs staining with anti-HMGB1 and SYTO13. Inhalation of NO did not influence these findings. Together, these studies show that LPS can increase levels of blood MPs and influence phenotype, including nuclear content. As such, particles may be a source of HMGB1 and other nuclear molecules in the blood during inflammation.