Enzyme immunoassay for the qualitative and quantitative determination of IgA antibodies against ß2- glycoprotein 1 in human serum. The anti-phospholipid syndrome (APS) is an autoimmune disorder which can comprise clinical conditions as venous and arterial thrombosis, thrombocytopenia, myocardial infarction, recurrent spontaneous abortion and neurological complications. Cardiolipin (CL) is the most common, negatively charged, acid phospholipid and autoantibodies to cardiolipin are characteristic of APS. Anti-cardiolipin autoantibodies are also present in some patients with SLE and related diseases which is typical for a secondary anti-phopholipid syndrome. Autoantibodies associated with APS are directed not only against CL and similar phospholipides but also against phospholipid/protein complexes. ß2-glycoprotein 1 (ß2-GP1; apolipoprotein H) has been identified as such a natural and essential co-antigen for CL-autoantibodies. It has been shown that ß2-GP1 autoantibodies are associated with a higher risk of thrombosis. Autoantibodies to ß2-GP1 also occur independently from those to cardiolipin.