Autoantibodies directed to negatively charged phospholipids, in particular cardiolipin, have been detected in the serum of patients with systemic lupus erythematosus (SLE) and antiphospholipid syndrome (APS). APS is characterised by venous and arterial thrombosis, recurrent spontaneous abortions and thrombocytopenia. It is now known that a serum cofactor, β2-glycoprotein 1, is required for the binding of cardiolipin by autoantibodies in the sera of patients with APS. By contrast, anticardiolipin antibodies from patients with infectious diseases (in particular, syphilis) do not require this cofactor. β2-Glycoprotein 1, also known as apolipoprotein H, is a relatively abundant serum protein (present at a concentration of about 0.2 mg/ml) that may play a role in coagulation. It has been shown to bind to platelets, mitochondria and negatively charged substances such as heparin, DNA, dextran sulphate and negatively charged phospholipids.