Beta-2-glycoprotein I (beta2GPI), a 50-kDA serum glycoprotein that binds negatively charged phospholipids plays a role in coagulation, thrombosis, and the clearance of phosphatidylserine expressing cells. Because of its recently recognized role in several autoimmune responses, we have developed a method that quantifies plasma beta2GPI levels by using a competitive ELISA assay. When combined with data from a standard ELISA, this method determines the concentration of free beta2GPI and the fraction of antibody-bound beta2GPI thereby facilitating quantification of total antigen in individuals with autoimmune antibodies. Standard competitive inhibition ELISA was compared with this method, which uses known amounts of standard beta2GPI added to the plasma as an internal standard. Identical results were obtained with both methods for plasma samples from normal individuals that did not contain blocking antibodies. Analysis of plasma from antiphospholipid syndrome patients (patients with autoantibodies to beta2GPI) by the internal standard method, however, resulted in significantly lower apparent beta2GPI levels indicating that a substantial fraction of the plasma beta2GPI was bound by antibody.