Released FXIII-A is cleaved by thrombin, whereas platelet-derived FXIII-A that remains associated with the platelet is protected from thrombin cleavage. (A) Quantification of relative FXIII-A⁽°⁾ and FXIII-A∗ in the releasate of CVX plus thrombin (IIa)–stimulated platelets as in Figure 2A-C. The data show mean ± SEM of 5 separate donors. (B) Representative immunoblot of FXIII-A in pellets (P) and releasate (R) from experiments with washed platelets from F13a1^+/+ or F13a1^−/− mice unstimulated or stimulated with CVX plus IIa in the absence (for F13a1^+/+ platelets) or presence (for F13a1^−/− platelets) of exogenous mouse FXIII (mFXIII-A₂B₂) for 30 minutes (n = 3). Proteins from ∼1.5 × 10⁶ platelets were loaded. FXIII-A species are labeled on the right of the immunoblot as ^(O) (representing zymogen FXIII-A or nonproteolytically activated FXIII-A°) and ∗ (representing FXIII-A∗).