Binding of fluorescein-labeled C1C2-2296C (C1C2*) to platelets before (center panel) and after (bottom panel) activation. The results are consistent with a smaller population of lower-affinity C1C2-binding sites on platelets prior to activation, whereas more than 90% of activated platelets became labeled after addition of 0.25 μM C1C2*. The upper-left panel shows the platelet control without added C1C2. Percentages are as in previous figures.