Platelet FXIII-A is associated with the membrane and cytoplasmic fractions. (A) Platelets (plt) were lysed or fractionated and FXIII-A activity (IU/mL) was measured in each fraction at the equivalent concentration (5 × 10⁸ plts/mL). Data are shown as mean with standard error of the mean, n = 8. (B) Platelets were fractionated and FXIII-A and FXIII-B antigens were detected in fractions by western blotting. Image shown is representative of 3 experiments. (C) (△) FXIII-depleted thrombi were prepared with (○) unstimulated lysed platelets at 5 × 10⁸ plts/mL, (▲) isolated cytoplasm from unstimulated platelets at 5 × 10⁸ plts/mL, (●) isolated membranes from unstimulated platelets at 5 × 10⁸ plts/mL, or (□) releasate collected from platelets (5 × 10⁸/mL) stimulated with 200 μg/mL collagen and 200 μM TRAP-6. **P < .01; ***P < .001 vs FXIII-depleted thrombi (n = 3).