Anti–β₂-GPI does not affect APC-Pt z-carboxyglutamic acid (Gla) activity but does inhibit APC/Pt1-22 activity. PE/PS/PC vesicles were either not oxidized (▪) or oxidized (▦) and used in clotting assays in the presence or absence of 250 nM anti–β₂-GPI mAb no. 1522 and anticoagulant enzyme as indicated on the x-axis. Similar results were obtained with APC and APC-PtGla with the other antibodies. Error bars indicate the SE of 2 determinations run in duplicate.