Binding of biotinylated C1C2–2296C (C1C2^b) and fluorescein-labeled C1C2-2296C (C1C2*) to platelets. (A) Binding of C1C2^b to platelets was detected by PE-streptavidin (FL2 on the y-axis is relative fluorescence using the channel for phycoerythrin, PE). The left panel shows platelets with PE-streptavidin added as a control. The right panel shows platelets after addition of 1.2 μM C1C2^b followed by PE-streptavidin. (B) Binding of fluorescein-labeled C1C2-2296C. The left panel shows the platelet-only control. The right panel shows platelets after addition of 0.25 μM C1C2* (FL1, y-axis). Note the reduced scatter and higher percentage of platelets binding compared with panel A, although only one fifth as much labeled C1C2 was used as in panel A. Shown are percentages of platelets binding C1C2^b-PE-streptavidin or C1C2* above the background fluorescence signal from platelets without the fluorescent probe.