Agonist-induced actin polymerization is abolished in ADF/n-cofilin–deficient platelets. After activation of washed platelets with thrombin (1 U/mL for 2 minutes at 37°C), platelets were fixed, permeabilized, stained with phalloidin-FITC, and analyzed by flow cytometry. The mean fluorescence intensity of resting control platelets was set to 1. (Left panel) The mean fluorescence intensity of resting and activated platelets was measured. (Right panel) The ratio of polymerized actin in activated vs resting platelets was determined. *P < .05; ***P < .001.