Binding of fluorescein-labeled FVIIIa (FVIIIa*) to activated platelets. From 0.2 to 20 nM FVIIIa* was added to activated platelets. Maximal binding occurred at approximately 10 nM FVIIIa, and half-maximal binding gave an estimated K_d of approximately 2.5 nM. Similar results were obtained for binding of fluorescein-labeled FVIII to activated platelets (not shown). Error bars represent SD.