Adherence of red blood cells to the endothelium initiates vaso-occlusion in sickle cell anemia. The increased adhesiveness of sickle erythrocytes is accompanied by several changes in the lipids of the erythrocyte membrane, including increased expression of phosphatidylserine (PS). One important PS-binding protein is lactadherin (also known as milk fat globule-EGF factor 8), a 45-kDa glycoprotein containing an Arg-Gly-Asp (RGD) sequence. Lactadherin is secreted by many cell types, including macrophages, and is known to have a role in the clearance of apoptotic lymphocytes by binding PS on the cell surface and thus presenting a binding site for macrophage integrins, including αvβ3. We recently showed that lactadherin, added exogenously to both normal and sickle erythrocytes, enhances phagocytosis of these cells by macrophages. Here, we investigated the effect of bovine lactadherin on the adhesion of sickle erythrocytes to the endothelium in flowing blood. Lactadherin (100 nM) promoted the adhesion of sickle erythrocytes to histamine-stimulated human umbilical vein endothelial cells (HUVECs) at shear stresses from 2.5 to 10 dyne/cm2. Adhesion was inhibited significantly by abciximab (10μ g/ml), an antibody against αvβ3 (P<0.05). Normal erythrocytes were induced to adhere to stimulated HUVEC in a lactadherin-dependent manner by treatment with 10 mM N-ethylmaleamide and 4 μ M calcium ionophore A23187—treatment that exposes phosphatidylserine on the erythrocyte surface. These results indicate that lactadherin mediates sickle cell adhesion to the endothelium by bridging PS on erythrocytes to αvβ3 integrin on the endothelium. We propose that sickle erythrocytes acquire lactadherin as they traverse macrophage-rich zones such as spleen, liver, and lymph nodes. Those that are not ingested immediately will become more adhesive for endothelium. Thus, lactadherin appears to be involved in both hemolysis and vaso-occlusion in sickle cell anemia.