IL3 exerts its biologic activity through its interaction with a cell surface receptor that consists of two subunits. The a subunit (CD123) specifically binds IL3, whereas the ß subunit is required for signaling and is common to the GMCSFR and IL5-R. 107D2.08 and 106C2.02 mAbs were obtained after mouse immunization with sorted human tonsillar PDC. Both clones strongly stain PDCs and basophils, weakly stain monocytes, CD34+ derived DCs and CD11c+ DC, while no staining is observed on T, B, NK cells as well as on mono-derived DCs. Staining with 107D2.08 and 106C2.02 mAbs are maintained on sorted PDC cultured in the presence of IL3 and CD40L, but lost when IL3 alone is added to the culture. The recognition of the IL3Ra chain by 107D2.08 and 106C2.02 was confirmed by transfection studies. 107D2.08 appeared to be the most appropriate clone for in situ studies. 107D2.08 allowed the first observation of IL3Ra+ cells in breast tumor microenvironment