Retention and release of FXIII-A from convulxin plus thrombin–stimulated platelets do not depend on endogenous fibrinogen or plasminogen. Washed platelets from fibrinogen-deficient (Fga^−/−), plasminogen- deficient (Plg^−/−), or littermate control (Fga^+/− and Plg^+/−, respectively) mice were unstimulated (Unstim) or stimulated with CVX plus thrombin (IIa) or A23187 for 30 minutes. The platelet pellets and releasate were separated by centrifugation. FXIII-A was visualized by immunoblotting and quantified by densitometry. (A-C) Studies with fibrinogen-sufficient and -deficient platelets. (D-F) Studies with plasminogen-sufficient and -deficient platelets. (A,D) Immunoblots of FXIII-A in the pellet or releasate from 1.5 × 10⁶ mouse platelets (mouse [m]FXIII-A₂B₂ loading control). The band in the MWM lane indicates 100 kDa. 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∗). (B,E) Quantification of total FXIII-A in the pellet. (C,F) Quantification of total FXIII-A in the releasate. The data show mean ± SEM; each dot is a separate mouse.