Beta 2-GPI is a single chain, 50 kd glycoprotein made up of 326 amino-acids, present in human plasma at concentrations of about 200 micrograms/ml. It has been shown to represent the "serum cofactor" important in determining the binding of aCL to phospholipids both in fluid and in solid phase assays. Its cofactor activity is demonstrable for aPL antibodies from patients with autoimmune but not with infectious diseases. It is immunogenic in heterospecific models, and immunization with beta 2-GPI seems to be able to induce the production not only of anti-beta 2-GPI but also of aPL antibodies. Most probably its binding to CL changes the configuration of phospholipids to a more immunogenic one. The various characteristics of beta 2-GPI are summarized in Table I. beta 2-GPI has been recognized as a natural anticoagulant protein, but its possible role in the pathogenesis of the APLS remains to be determined.