['The activation marker CD40 ligand (CD40L) has recently been demonstrated to be released from the cytoplasm of platelets (PLTs) during storage. CD40L may be associated with some adverse transfusion reactions including febrile responses and transfusion-related acute lung injury. CD62P has been traditionally measured to assess PLT activation. This study compares the surface levels of CD40L and CD62P and accumulation of the soluble forms of these activation markers in the plasma of stored PLTs, prepared by PLT-rich plasma (PRP) or buffy coat (BC) methods and with two apheresis instruments.', 'Individual PLT concentrates (PCs) were prepared in 100 percent plasma from a pool of two ABO-identical whole-blood units. Apheresis PLTs (APs) were prepared in 100 percent plasma using one of two commercially available cell separators (Amicus, Baxter Healthcare; and Trima, Gambro BCT). Surface expression of CD40L and CD62P was measured by flow cytometry, and secretion of soluble CD40L (sCD40L) and soluble CD62 (sCD62) was measured by enzyme-linked immunosorbent assay during 7 days of PLT storage.', 'Secretion of sCD40L was greater in Amicus APs than in Trima APs during the first 3 days of storage. It was also greater in the PRP-PC preparations than in BC-PC preparations through the first day of storage. Surface expression of CD40L was low in all PLT preparations. Secretion of sCD62P was greater in Amicus APs than in Trima APs during the entire storage period and greater in PRP-PC than in BC during the first 5 days of storage. The percentage of CD62P-positive PLTs was greater in Amicus units than Trima units and greater in PRP-PC than BC-PC preparations during the first 5 and 3 days of storage, respectively.'] Do platelet products prepared by different methods of sedimentation undergo platelet activation differently during storage?