Elevated plasma endothelial microparticles (EMP) have been documented in MS during exacerbation. However, the role of EMP in pathogenesis of MS remains unclear. We investigated the formation of EMP-monocyte conjugates (EMP-MoC) and their potential role in transendothelial migration of inflammatory cells in MS. EMP-MoC were assayed in 30 MS patients in exacerbation, 20 in remission and in 35 controls. EMP-leukocyte conjugation was investigated flowcytometrically by employing α-CD54 or α-CD62E for EMP, and α-CD45 for leukocytes. EMP-MoC were characterized by identifying adhesion molecules involved and their effect on monocyte function. In vivo (clinical): EMP-MoC were markedly elevated in exacerbation vs. remission and controls, correlating with presence of GD+ MRI lesions. Free CD54+ EMP were not elevated but free CD62E+ EMP were. In vitro: EMP bound preferentially to monocytes, less to neutrophils, but little to lymphocytes. Bound EMP activated monocytes: CD11b expression increased 50% and migration through cerebral endothelial cell layer increased 2.6-fold. Blockade of CD54 reduced binding by 80%. Most CD54+ EMP bound to monocytes, leaving little free EMP, while CD62+ EMP were found both free and bound. These results demonstrated that phenotypic subsets of EMP interacted differently with monocytes. Based on our observations, EMP may enhance inflammation and increase transendothelial migration of monocytes in MS by binding to and activating monocytes through CD54. EMP-MoC were markedly increased in MS patients in exacerbation compared to remission and may serve as a sensitive marker of MS disease activity.