Platelets contain a pool of endogenous platelet-von Willebrand factor (vWF) that becomes expressed on the platelet surface when platelets are stimulated by a variety of agonists. Maximal platelet-vWF expression occurs in concert with platelet alpha-granule secretion. Aspirin (ASA) is known to impair platelet activation and alpha-granule secretion by irreversible inhibition of platelet cyclo-oxygenase. We studied native and ASA-treated platelets for their ability to mobilize and to express platelet-vWF in response to adenosine diphosphate (ADP) or thrombin. We found that each agonist was effective in promoting increased platelet- vWF surface expression on native and ASA-treated platelets. ASA-treated platelets responded identically to native platelets to low (0.01 U/mL) and high (1.0 U/mL) concentrations of thrombin, while the ADP-induced increase in ASA-treated platelets was only 50% to 60% of that for control platelets. Measurement of secreted platelet-vWF and beta- thromboglobulin indicated that the increase seen with ADP was largely independent of alpha-granule secretion. Using monoclonal antibodies (MoAbs) against the platelet glycoproteins (GP) IIb/IIIa and Ib (MoAbs 10E5 and 6D1, respectively), we demonstrated that the ADP-induced increase in platelet-vWF expression on control platelets primarily involved the binding of secreted platelet-vWF to the platelet GPIIb/IIIa. In contrast, the increase in platelet-vWF that occurred following ADP stimulation of ASA-treated platelets was largely insensitive to GPIIb/IIIa blockade. No effect of GPIb blockade in platelet-vWf expression was noted for either control or ASA-treated platelets. When platelet shape change was prevented by the addition of cytochalasin D, ADP-induced platelet-vWf surface expression on ASA- treated platelets was reduced by more than 80%. Our data indicate that platelets in which the cyclooxygenase pathway is blocked by the action of aspirin can increase surface expression of platelet-vWf as a consequence of platelet shape change. We speculate that this process exposes platelet-vWf bound to GPIIb/IIIa, or possibly GPIb, within the surface connected canalicular system.