Inhibition of APC inactivation of factor Va by anti–β₂-GPI requires oxidized phospholipid in purified systems. Factor Va (50 nM) was reacted with APC (2.5 pM) on nonoxidized (left panels) or oxidized (right panels) phospholipid vesicles in the absence (top panels) or presence (bottom panels) of anti–β₂-GPI no. 1522 (1 μM) for the times indicated. Factor Va remaining (y-axis) was determined by clotting assay in factor V–deficient plasma as described in “Materials and methods.”× indicates no other additions; ▴, + protein S; ○, +β₂-GPI; □, + protein S +β₂-GPI. Similar results were obtained with mAbs 1519 and 1529.