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We investigated the kinetics of plasma concentrations of endogenous erythropoietin (EPO), tumor necrosis factor-α (TNF) and interleukin-6 (IL-6) in patients with metastatic breast cancer who received high-dose chemotherapy and peripheral blood stem cell transplantation (PBSCT) followed by an administration of recombinant human granulocyte-colony-stimulating factor (rhG-CSF). The plasma levels of these cytokines increased significantly after PBSCT and peak levels were observed on day 6 (median) for EPO, day 14.5 for TNF and day 6 for IL-6. EPO levels inversely correlated with hemoglobin levels (p = 0.0207) but no correlation was found between neutrophil recovery and levels of the cytokines tested. The concentrations of TNF and IL-6 directly correlated with body temperature (p = 0.0470 and <0.0001). The maximum level of IL-6 which occurred within 10 days after PBSCT directly correlated with number of days to achieve a platelet count of >20 X 10(9)/1 (p = 0.0003) and number of platelet transfusions (p = 0.0107), suggesting that the high concentrations of IL-6 could be associated with delayed platelet recovery.