Abstract: FL (flt3 ligand)is an early acting hematopoietic cytokine.By binding and activating its specific tyro-sine kinase receptor flt3,FL can stimulate the proliferation and differentiation of hematopoietic stem and progeni-tor cells.Recently,it has been shown that the level of FL in plasma of patients with bone marrow aplasia in-creases dramatically.Plasma level of FL also can be elevated and is a possible bio-indicator for bone marrow aplasia in radiation-induced damage.Furthermore,FL,alone or in combination with other cytokines,provides protection from bone marrow aplasia after radiation.