Antiphospholipid (aPL) antibodies are of major interest not only because the lupus anticoagulant (LA) causes an inhibition of in vitro blood coagulation, but also because the presence of aPL antibodies confers a risk of thrombosis. The inhibition of in vitro phospholipid- dependent coagulation (LA) is thought to be caused by the binding of LA to procoagulant phospholipid surfaces, thus impeding the clotting process. Another class of aPL antibodies are those originally described to be directed against negatively charged phospholipids, in particular cardiolipin (ACA). ACA are usually directed against a complex antigen consisting of negatively charged phospholipid and a plasma protein, beta 2-glycoprotein I (beta 2-GPI). Further, there is antibody heterogeneity even within individual patients so that ACA and LA are separable using physicochemical techniques such as ion exchange chromatography and chromatofocusing. Using such techniques we have enriched Ig fractions for LA and ACA from two patient plasmas. The majority of Ig with LA activity had a pI of 7.2 to 7.3 whereas ACA had a pI of 5.0 to 5.2. Using these enriched fractions labeled with [125I]- iodine we have shown that LA binds to platelets in a specific and saturable manner. Binding is dependent on thrombin activation. [125I]- ACA behaves differently. Like LA, binding is specific and dependent on thrombin activation but in this case requires the presence of beta 2- GPI. ACA, in the presence of beta 2-GPI, competes for binding with LA suggesting the same or contiguous site. There is no cross-reactivity of these antibodies with GPIIb/IIIa and the most likely binding site is phospholipid. In neither case does LA nor ACA have an effect on thrombin-induced release of serotonin or beta-thromboglobulin nor do they affect platelet aggregation induced by a number of agonists. This antibody binding may play an etiological role in thrombocytopenia associated with aPL, but does not explain thrombosis on the basis of hyperaggregability or increased platelet release.