Collagen-induced degranulation of platelets is inhibited by the ADP receptor antagonists. Stimulation of whole blood for 10 minutes at 37°C with collagen (coll; 10 μg/mL) elicits the exposure of P-selectin. In additional samples, MRS-2179 (1 mM) and AR-C69931MX (10 μM) plus vehicle were added. Flow cytometric detection using the labeled anti–P-selectin antibody. Co indicates control. Means ± SDs; n = 3. *P < .05 (versus collagen alone).