Murine platelets require 12-LOX for normal FcγRIIa-induced platelet aggregation. A dose response of anti-CD9–induced platelet aggregation was performed with washed platelets from hFcR/ALOX12^+/+ or hFcR/ALOX12^−/− mice. Prior to aggregation, fibrinogen (75 μg/mL) and CaCl₂ (1 mM) were added to platelets. (A) Washed platelets from hFcR/ALOX12^+/+ (gray tracings) and hFcR/ALOX12^−/− (black tracings) were measured for aggregation in response to 0.25, 0.5, and 1 μg/mL of mouse anti-CD9 for 10 minutes. Inset: western blots for 12-LOX, FcγRIIa, and GAPDH were performed with platelet lysate from hFcR/ALOX12^+/+ or hFcR/ALOX12^−/− mice. The lag time (B) and final aggregation (C) was measured in hFcR/ALOX12^+/+ (gray bars) and hFcR/ALOX12^−/− (black bars) washed platelets stimulated with increasing doses of anti-CD9 (n = 3 to 6 per group). Error bars indicate SEM. *P < .05.