Various cells such as platelets, lymphocytes, endothelial cells, red blood cells and monocytes do release surface-derived microparticles (mps). We analysed mp isolated from supernatants of cultured antigen-presenting human cells (APCs) and human cell lines. Particle sizing by dynamic light scattering revealed a characteristic size of the particles ranging from 80 nm to 300 nm in viable cells and from 400 nm to 1200 nm in irradiated cells. Employing flow-cytometry, we observed partly an altered surface protein composition of the mp compared to their cellular source. Mp originating from dendritic cells (DCs) differed in their surface composition from those released from monocytes and monocyte-derived macrophages. In functional assays, these mp stimulated alloreactive T-cells. The treatment of the cells with either UV-B or lipopolysaccharide strongly influenced the quantity, the immunostimulatory features and the surface composition of the mp. Mp from apoptotic macrophages were able to reduce the stimulatory capacity of vital macrophages but not of DC. Apoptotic mps from DC, on the other hand, were always stimulatory. This is the first report regarding the study of mp released from DC and compared with those released from other APC.