The monoclonal antibody FA-11 reacts with murine macrosialin (mouse CD68), a heavily glycosylated transmembrane protein of 87- 115 kDa, which is specifically expressed by tissue macrophages, Langerhans cells and at low levels by dendritic cells. Macrosialin belongs to the lysosomal-associated membrane protein (LAMP) family. In common with the LAMPs, macrosialin is a type I me.mbrane protein, containing a short and highly conserved cytoplasmic tail, followed by a transmembrane domain that precedes the intraluminal region. In macrophages, macrosialin is mainly expressed as a late endosomal protein and rapidly exchanges with a small subset of macrosialin present on the cell surface. Several reports have shown that macrosialin recognises oxidized low-density lipoproteins as well as the intercellular adhesive molecule (ICAM-L) raising the possibility of a receptor function of this protein. In human, macrosialin has been suggested to be a novel prognostic factor for classical Hodgkin’s lymphoma- since high expression of macrosialin and CD163 correlates with adverse outcome.The monoclonal antibody FA-11 detects surface macrosialin at low levels in resident mouse peritoneal macrophages which can be enhanced by thiolycollate stimulation. Macrosialin is predominantly located within the cell and can be detected by flow cytometry with the monoclonal antibody FA-11 when cell permeabilisation is used.