We previously described a mouse line that contains green myelomonocytic cells due to the knock-in of enhanced green fluorescence protein (EGFP) into the lysozyme M gene. We have now created a transgenic line with fluorescent erythroid cells using a β-globin locus control region driving the enhanced cyan fluorescence protein (ECFP) gene. These mice exhibit cyan fluorescent cells specifically in the erythroid compartment and in megakaryocyte-erythroid progenitors. Crossing the animals with lysozyme EGFP mice yielded a line in which live erythroid and myeloid cells can readily be distinguished by fluorescence microscopy and by fluorescence-activated cell-sorter scanner. This cross allowed unambiguous identification of unstained mixed erythroid-myeloid colonies for the first time. The new mouse lines should become useful tools to dissect the branching between erythroid and myelomonocytic cells during in vitro differentiation of definitive multipotent progenitors. © 2003 by The American Society of Hematology.