Background: We previously found plasma levels of CD40 ligand (CD40L), chemokine (C-X-C motif) ligand 5 (CXCL5), chemokine (C-C motif) ligand 5 (CCL5), and epidermal growth factor (EGF) to be low in aplastic anemia (AA) patients and to be correlated with the platelet count. Objectives: To study the association of CD40L, CXCL5, CCL5, and EGF with platelets. Patients/Methods: We measured cytokines in the plasma of immune thrombocytopenic purpura (ITP) and AA patients using the Luminex assay and confirmed the results in a mouse model and in vitro experiments. Results: Both ITP and AA showed similarly low levels of CD40L, CXCL5, CCL5, and EGF, compared with healthy controls. In ITP, levels of these proteins were significantly greater in patients with higher platelet counts than in those with lower platelet counts. In a murine thrombocytopenia model, levels of CD40L, CXCL5, CCL5, and EGF decreased with platelet count after immune-mediated destruction, while the cytokine levels increased when the platelet count recovered. In vitro, concentrations of these cytokines in the supernatants of platelet suspensions were proportional to platelet numbers, and levels in sera prepared by simple blood coagulation were equivalent to those in platelet-rich plasma-converted sera. mRNA expression for CXCL5, CCL5, and EGF was higher in platelets than in megakaryocytes, peripheral blood mononuclear cells, granulocytes, and non-megakaryocytic bone marrow cells. Conclusions: Plasma CD40L, CXCL5, CCL5, and EGF are mainly platelet-derived, suggesting a role of platelets in immune responses and inflammation. Measurement of CD40L, CXCL5, CCL5, and EGF in human blood allowed testable inferences concerning physiology and pathophysiology in quantitative platelet disorders. © 2012 International Society on Thrombosis and Haemostasis.