Apolipoprotein-H, also known as Beta2-Glycoprotein I (B2GPI), is a plasma glycoprotein that circulates at a concentration of 200 ug/ml (4 micrometers). Synthesized in the liver, Beta2-Glycoprotein I is a single chain molecule of 48 kDa, consisting of five repeating internally disulphide-bonded structures referred to as sushi domains. Relative to other glycoproteins, Beta2-Glycoprotein I has an unusually high content of cysteine (6.2%), proline (8.3%) and carbohydrate (19%). Almost half the circulating Beta2-Glycoprotein I in plasma is associated with lipoproteins of all major fractions. Beta2-Glycoprotein I has been demonstrated to bind negatively charged phospholipids, heparin and platelets. Although the precise function(s) are as yet unknown, Beta2-Glycoprotein I has been demonstrated to interfere with blood coagulation by competitively binding to negatively charged phospholipid surfaces exposed during cell activation or damage. Recent evidence also implicates Beta2-Glycoprotein I as a cofactor recognized by anti-phospholipid antibodies present in some autoimmune disorders such as systemic lupus erythematosus (SLE)^1-3.