Reversibility of the accumulation of FITC-annexin V on the surface of single, collagen-bound platelets. Platelets were allowed to adhere to immobilized collagen in the presence of 2 mmol/L CaCl₂ and FITC-labeled annexin V (A). After 15 minutes of adhesion, 10 mmol/L EGTA was added to displace annexin V from the platelet surface (B). Changes in FITC-fluorescence intensity of single platelets were recorded by video fluorescence imaging techniques. Time settings are relative to the timepoints of interaction of the platelets with collagen.