A method for the extraction and purification of PrPC, in its native monomeric form, from outdated human platelet concentrates is described. Both calcium ionophore platelet activation and lysis in Triton X-100 were evaluated as methods for the extraction of soluble platelet PrPC in its monomeric form. Following platelet activation, the majority of released PrPC was detected as a disulphide linked high molecular weight complex, which under reducing conditions could be separated into what appear to be stable non-disulphide linked PrP dimers or PrP covalently linked to another as yet unidentified protein. This phenomenon appears to be unique to activation since only monomeric PrPC was detected following lysis of resting platelets. Subsequently, PrPC was purified from the Triton X-100 lysate by sequential cation ion exchange and Cu2+ affinity chromatography. From 10 L of outdated platelet concentrate, we were able to recover 1.29 mg PrPC at a purity of 92%. (c) 2005 Elsevier Inc. All rights reserved.