The novel immunomodulator FTY720 causes lymphocytes from peripheral blood to accumulate in lymphoid tissues. In vivo, FTY720 is phosphorylated to FTY720-P, which binds to the sphingosine 1-phosphate receptor S1P1. So far, it has been unclear where FTY720-P is produced. We demonstrate that platelets efficiently convert FTY720 to FTY720-P and release it into the extracellular space. This release is mostly independent of platelet activation, but is slightly increased upon thrombin stimulation. These results suggest that platelets are a major source of plasma FTY720-P, and that FTY720-P release is mediated by two different transporters.