BACKGROUND To identify the mechanisms of the hypercoagulability associated with antiphospholipid antibodies, we investigated antibody-mediated platelet activation and interference of antibodies with phospholipid-dependent reactions. METHODS We used two murine monoclonal antibodies, one against beta(2)-glycoprotein I (7F6G), the other against prothrombin (28F4). Platelet activation was assessed by phospholipid-related platelet procoagulant activity. Endogenous thrombin potential without activated protein C (ETP(0)) and the activated protein C concentration that reduced the ETP(0) by 50% (IC(50)-APC) were determined by calibrated automated thrombography. RESULTS Both monoclonal antibodies mimicked the effect of IgG in 11 out of a series of 40 patients with antiphospholipid antibodies in thrombography. In the presence of their target, 7F6G and 28F4 at 200 microg/mL exhibited comparatively low and high binding to platelets and elicited low and high levels of procoagulant phospholipids on platelet surface, respectively. In platelet-poor plasma, these antibodies induced a 1.6 and >12-fold increase in IC(50)-APC, respectively, thus providing evidence for a procoagulant effect independent of platelet activation. The 84% decrease in ETP(0) indicated that 28F4 also displayed an anticoagulant effect. In platelet-rich plasma, this anticoagulant effect was significantly less (23% decrease in ETP(0)), demonstrating that a high increase in procoagulant surfaces by platelet activation significantly antagonizes the anticoagulant effect of antiphospholipid antibodies. In both types of plasma, the inhibition of thrombin generation (reduced ETP(0)) was less than the inhibition of activated protein C activity (increased IC(50)-APC). CONCLUSIONS Our findings show that platelet activation reinforces the hypercoagulability induced by competition between antiphospholipid antibodies/target complexes and pro- and anticoagulant complexes for phospholipid surfaces.