Fms-related tyrosine kinase 3-ligand (Flt3L) is a non-redundant role for dendritic cell development. Because both Flt3L-/- and its receptor, Flt3-/- mice showed massive reduction of plasmacytoid dendritic cell (pDC) and conventional DC in the lymphoid organs. Injections and over expression of Flt3L leads to massive expansion of all DC subsets. Overexpression of Flt3 in Flt3-negative progenitor rescue DC developmental potential. Thus, Flt3L is a critical and instructing cytokine for DC development and homeostasis. In this study, we identify Flt3L expressing cell in vivo by using Flt3L-reporter mice. To visualize Flt3L expressing cell, we generated transgenic mice carrying mCherry reporter gene under the control of Flt3L promoter. The level of Flt3L-mCherry expression was high in both CD4+ and CD8+ T cells, followed by NKT cells and NK cells, and low in B cells in the spleen. Among CD4+ T cells, the level of Flt3L-mCherry was higher in memory CD4+ T cells, compared with naive CD4+ T cells. Other hematopoietic cells, such as DC subsets, monocytes, granulocytes, and erythrocytes showed quite low level of Flt3L-mCherry expression, and niche cells, such as CAR cells, mesenchymal stem cells, and osteoblasts in the BM hardly expressed Flt3L-mCherry. The results were confirmed by immunohistochemical analysis. Since DC development is initiated in the BM, these results imply that memory CD4+ T cell-derived Flt3L act on common DC progenitor (CDPs) to induce DC development.