Anti-cardiolipin autoantibodies (aCL) induce thrombosis and recurrent fetal death. These antibodies require a 'cofactor', identified as beta 2-glycoprotein I (beta 2-GPI), to bind phospholipids. We show here that aCL can bind beta 2-GPI in the absence of phospholipid. Binding of aCL to beta 2-GPI is dependent upon the beta 2-GPI being immobilized on an appropriate surface including cardiolipin, irradiated polystyrene and nitrocellulose membrane. This effect cannot be explained by increased antigen density of beta 2-GPI immobilized on these surfaces. Rather, conformational changes that occur following the interaction of beta 2-GPI with phospholipid render this protein antigenic to aCL. Liquid-phase beta 2-GPI was not antigenic for aCL. Thus, aCL cannot bind circulating beta 2-GPI. These findings may explain why patients with aCL can remain healthy for many years but then undergo episodes of thrombosis or fetal loss without changes in their circulating aCL profile, as the triggering event for these pathologies can be predicted to be one that renders beta 2-GPI antigenic for aCL.