Estimated number of fluorescein-labeled C1C2 (C1C2*)–binding sites per platelet. The top histogram shows fluorescent signals from a mixture of 7.6 μm beads, where 1 is the peak for unlabeled beads and 2 through 5 are signals from the same beads containing increasing, defined amounts of FITC label. The bottom panel shows a histogram representation of near-saturation binding of 0.25 μM C1C2* to activated platelets. From 3 sets of similar measurements using different platelet preparations, the estimate ranged from 6100 to 7600 C1C2 molecules bound per activated platelet.