BACKGROUND Platelets contain several mediators, belonging to a family of proinflammatory cytokines named chemokines, that are stored in the organelles. Release and accumulation of these chemokines during storage of platelet concentrates (PCs) might be responsible for nonhemolytic transfusion reactions. METHODS Analysis was done of pH and the levels of platelet factor 4, beta-thromboglobulin, interleukin 8, RANTES, macrophage-inflammatory protein-1 alpha, lactate dehydrogenase, and serotonin in the supernatant of stored PCs on Days 1, 3, 5, and 8. PCs were prepared by apheresis or from pools of four buffy coats. Buffy coat PCs were filtered before storage. RESULTS Nonfiltered apheresis PCs, which had a higher white cell contamination (p < 0.01), contained significantly more platelets than did buffy coat PCs (p = 0.02). The pH decreased significantly in apheresis PCs (p = 0.01), whereas there was a significant increase in lactate dehydrogenase (p < 0.001). In buffy coat PCs, pH remained stable and lactate dehydrogenase increased moderately. Concentrations of platelet factor 4 and beta-thromboglobulin increased steadily in both preparations over the storage period (p < 0.001). Macrophage-inflammatory protein-1 alpha was hardly detectable in the supernatant of both PCs, while RANTES levels increased significantly with storage time (p < 0.001). Interleukin 8 was not found in the supernatant of any PCs, with the exception of one apheresis PC with high white cell contamination (> 10(9)/ L). Serotonin levels were higher in apheresis PCs (p = 0.01), but the levels did not correlate with storage time. CONCLUSIONS Platelet factor 4, beta-thromboglobulin, and RANTES were released from platelets during storage and accumulated over time in the PCs. These chemokines might play a causative role in nonhemolytic transfusion reactions because of their inflammatory potential, but the clinical effects of the transfusion of PCs with high chemokine contents remain to be investigated.