APC and heparin abolish the platelet-activating potential of histones. Histones (40 μg/mL) pretreated with APC (6 μg/mL) or heparin (1 U/mL) were added to gel-filtered platelets as described in “Methods,” and then their platelet-activating potential was evaluated by measuring PS exposure (A), FV/Va (B), and P-selectin expression (C) by flow cytometry. Data are means ± SD, n = 3. *P < .05 compared with histones.