BACKGROUND: The platelet (PLT) storage lesion remains incompletely understood. To gain a greater insight into the PLT storage lesion, a proteomic analysis of supernatant from leukofiltered pooled buffy-coat PLT concentrates (PCs) was undertaken. STUDY DESIGN AND METHODS: PCs were prepared in PLT additive solution and stored according to standard blood bank procedures. Supernatant samples were collected throughout 7 days of storage. Maps of supernatant proteins were generated by two-dimensional (2D) gel electrophoresis and mass spectrometry. Cytokine antibody microarrays and enzyme-linked immunosorbent assay were used to investigate bioactive molecules. RESULTS: The 2D gel maps of PC supernatant proteins displayed many features of plasma protein maps. Several storage-induced protein changes were identified including modifications to major plasma proteins. PLT-derived proteins were also identified, including tremlike transcript 1 and integrin-linked kinase, which may influence PLT-endothelium interactions. Cytokine antibody microarrays revealed a number of bioactive proteins that have not been previously associated with PCs produced for transfusion, such as brain-derived neurotrophic factor (BDNF). The concentration of PLT-derived cytokines including BDNF, CXCL7, epidermal growth factor, PLT-derived growth factor (PDGF), and CCL5 significantly increased during storage of PCs. Extended storage from Day 5 to Day 7 caused significantly increased levels of BDNF, PDGF, and CCL5 in PC supernatant. CONCLUSION: Proteomic techniques provide valuable new insight into the effects of storage on PCs and the contribution of soluble proteins to the development of the PLT storage lesion and recipient responses to PLT transfusion.