In some patients, phospholipid binding antibodies have been associated with recurrent venous and/or arterial thrombosis as well as with recurrent fetal loss (the "Antiphospholipid Syndrome" (APS)). These antibodies are induced in a variety of clinical circumstances and it is likely that the above clinical associations are limited only to patients with particular subgroups of anti-phospholipid (aPL) antibodies. Antibody subgroups may differ in their affinities for particular phospholipids or in their specificities for particular phospholipid conformations, such conformations being determined by phospholipid-lipid mixtures. It is possible that manipulation of phospholipid antigen concentration or of phospholipid-lipid mixtures in an ELISA assay system may enable more specific identification of patients with the APS. With a view to preparing a marketable ELISA kit specifically designed to identify patients with the APS, this study proposes to compare ELISA assay systems with varying concentrations of negatively charged phospholipids and phospholipid-lipid mixtures, to determine the system which is most sensitive and specific for identifying APS patients. Materials used in the assay systems will be evaluated for their stability over time to determine their utility in an ELISA assay kit.