Fibrinogen, but not VWF, deficiency impaired P-selectin expression on the platelet surface. Gel-filtered platelets were incubated with either thrombin (left panels) or TRAP (middle panels), and P-selectin expression on the platelet surface was analyzed via flow cytometry (A-C). Right panels and Figure 1D indicate mean fluorescence intensity (MFI) ± SEM. (A) A significant decrease in platelet surface P-selectin expression was observed on thrombin- or TRAP-activated Fg/VWF^−/− platelets compared with wild-type controls (arrow indicated); n = 3 (**P < .001). No significant difference in P-selectin expression was observed on resting Fg/VWF^−/− platelets compared with wild type (shaded). (B) No significant difference in platelet P-selectin expression was observed on VWF^−/− platelets (black) compared with wild type (shaded); n = 3, P = .85 (thrombin), P = .24 (TRAP). (C) A significant decrease in P-selectin expression on the surface of activated Fg^−/− platelets (arrow indicated) was observed compared with wild-type littermate controls (shaded); n = 3 (**P < .001). No significant difference was observed between Fg^+/− and wild type; n = 3 (P > .05). (D) A significant decrease in P-selectin expression on the surface of A23187- and PMA- activated Fg^−/− platelets was observed compared with wild type; n = 3 (*P < .05, **P < .01, respectively).