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SummaryADP causes human, aspirin-treated, gel-filtered platelets to change from their native discoid shape to spiny spheres with pseudopods, bind 125I-labeled fibrinogen, and aggregate if shaken with sufficient fibrinogen. After destruction of the added ADP with the enzyme apyrase, the platelets revert to a disc shape and lose much of their bound fibrinogen. Colchicine (208 μM or 83 μg/ml) added to ADP-treated platelets before apyrase prevented restoration of the discoid shape but not the loss of bound fibrinogen. It did not inhibit ADP-induced shape change, aggregation, or fibrinogen binding. Cytochalasin B (0.02–0.2 μM or 0.01–0.10 μg/ml) prevented ADP-induced shape change but not ADP-induced fibrinogen binding or aggregation. Thus, these findings support earlier studies with thrombasthenic and EDTA-treated platelets and with normal platelets at low pH, or in the presence of EDTA to indicate that fibrinogen binding is associated with aggregability but not with platelet shape.