Cardiolipin binding of IgG-class anticardiolipin antibody (aCL) depends on the existence of beta 2-glycoprotein I (beta 2-GPI). We developed an EIA system that enables detection of antibodies against beta 2-GPI, without the presence of cardiolipin. This system involves use of irradiated polystyrene plates, in which oxygen atoms are introduced onto the surfaces of the plates. beta 2-GPI bound to the surface of these plates is assumed to undergo a conformational change that exposes normally cryptic epitopes. Anti-beta 2-GPI antibody measured using this EIA system showed good correlation with aCL measured by conventional EIA methods and may prove useful in evaluating the risk of thrombosis and monitoring the clinical course in patients with SLE. Utilizing this EIA system and beta 2-GPI-deleted mutants, we found that the fourth domain of beta 2-GPI is involved in expression of one of the cryptic epitopes recognized by aCL. We also found that oxidized LDL are sequentially targeted by beta 2-GPI and aCL.