Ex vivo evidence for role of P2Y₁₂ receptor in activation of blood-based TF. Healthy volunteers were supplemented for 4 days with clopidogrel (A,C) and, following a washout phase, for another 4 days with aspirin (B,D). The collagen-elicited TF exposure in association with monocytes (CD14⁺ cells) in whole blood was determined before (pre) and after (post) the 4-day period (A-B). Under similar conditions, platelets and neutrophils were isolated from the volunteers before (pre) and after (post) treatment with clopidogrel and aspirin, and the TF activity was assessed in suspensions of collagen-activated platelets and neutrophils (C-D). Means ± SDs; n = 8. *P < .05 (versus “pre”).