Low-dose agonist aggregation of platelets isolated from mPF4^–/–, hPF4⁺, and WT mice. Platelets isolated from mPF4^–/– and littermate WT animals were stimulated by the addition of either a high (0.5 U/mL) (A) or low (0.05 U/mL) concentration of thrombin (B). Platelets isolated from hPF4⁺ and littermate WT animals were stimulated by adding either a high (0.5 U/mL) (C) or low (0.05 U/mL) concentration of thrombin (D). Aggregations were performed at 37°C with continuous stirring, and light transmission was continuously recorded. Data presented are representative of 3 sets of studies with similar outcomes.