Beta2 Glycoprotein I (b2GPI) is a serum protein with a single polypeptide chain consisting of 326 amino acids and having a molecular weight of 50kD. The protein is also known by the alternative name Apolipoprotein H (ApoH) which gives an indication of its phospholipid binding function. Since the detection of 2 Glycoprotein I as a target of antiphospholipid antibodies in 1990 the discussion about the clinical relevance of beta2 GP I is still controversial. It is well established that anticardiolipin antibodies are a very heterogeneous group and that beta 2 GP I is only one of several potential targets for anticardiolipin antibodies. Nevertheless it is generally accepted that the binding of 2 GP I to anionic lipids or surface is essential for the differential detection of anticardiolipin antibodies associated with autoimmune diseases (versus those associated with infectious diseases like syphilis, tuberculosis, AIDS, etc). The close relationship of beta2-GP1 antibodies and cardiolipin antibodies may indicate that the b2 GP I isotypes have the same clinical relevance as the cardiolipin isotypes, where IgM, IgG and IgA autoantibodies have been observed and are routinely determined diagnostic targets. b2GP-1 antibodies and anticardiolipin antibodies can be detected in patients with arterial and venous thrombosis, recurrent fetal loss, thrombosis and SLE. The titers of anticardiolipin and anti-beta2 GP I antibodies usually fluctuate sufficiently that they cannot be used for disease monitoring.