FITC-fibrinogen binding to Syk null and Syk positive platelets. PRP from (▪) Syk positive or (□) Syk null mice was treated with the indicated platelet agonists, PBS (control), ADP (10 μmol/L) plus epinephrine (25 μmol/L), or PMA (20 μmol/L). FITC-labeled fibrinogen was added with the agonists. After 30 minutes of incubation in the dark at room temperature, samples were diluted in 0.5 mL Tyrodes buffer and analyzed on a FACScan. Bars represent the geometric mean ± standard error fluorescent channel for PAC-1 binding and were 2.75 ± 0.26 (control), 28.94 ± 2.75 (ADP + epi), and 38.31 ± 4.33 (PMA) for the Syk null mice (where n = 7), and 2.85 ± 0.22 (control), 42.4 ± 2.49 (ADP + epi), and 39.5 ± 2.89 (PMA) for Syk positive mice (where n = 12, including 10 mice repopulated with Syk^+/− liver cells and 2 repopulated with Syk^+/+ liver cells). *P = .0029 (Student’s t-test). Similar results were obtained with a different batch of radiation chimeras.