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Activation of platelets during collection and storage has been implicated as a major cause of the platelet storage lesion. In this study, we investigated the effect of an automated plateletpheresis procedure on the in vivo platelet activation in 20 volunteer donors. Peripheral blood samples were collected immediately before and after plateletpheresis on the Haemonetics V50 Blood Cell Separator. Activation of platelets was determined by quantitating the amount of platelet P-selectin (CD62) expression using a whole blood method on flow cytometry. Adenosine diphosphate (ADP), collagen, and ristocetin induced platelet aggregations were also measured on a whole blood impedance aggregometer. Plateletpheresis caused a significant decrease in the CD62-positive platelet percentage and aggregation responses to 3 agonists. We concluded that the plateletpheresis procedure did not cause an increase in platelet activation in donors. Further studies are required to elucidate whether activated platelets are collected during the procedure or removed from the circulation of the donor and replaced by resting platelets, activated platelets bind to leukocytes or endothelial cells, and the plateletpheresis procedure is a powerful stimulus for platelet activation.