Reversal of APTT prolongation in thrombin-infused monkeys with anti-APC monoclonal antibody C3. APTT was measured in plasma specimens drawn at the indicated time points under 3 conditions: (1) no addition (black bars); (2) in the presence of 40 μg/mL monoclonal antibody C3, a murine anti-APC IgG₁ that reverses the anticoagulant activity of APC (hatched bars); and (3) in the presence of 40 μg/mL irrelevant murine IgG₁ monoclonal antibody (open bars). Panel A shows vehicle-infused monkeys (n = 6); panel B, PF4-infused monkeys (n = 6). Data shown are means ± SEM.