Anticoagulant response to thrombin infusion in presence or absence of infused platelet factor 4. APTT was measured in plasma specimens at the indicated time points before, during, and after a 10-minute thrombin infusion (black bar under x-axis). Vehicle-infused monkeys (n = 6), □; PF4-infused monkeys (n = 6), ▪. The effect of PF4 infusion in the absence of thrombin infusion was assessed in a single monkey (▴). Data shown are means ± SEM.