Donor-derived erythroid cells have a selective advantage over endogenous CA erythroid cells. (A) GFP⁺ donor erythroid cells were quantified by flow cytometry in the bone marrow and spleen of several CA recipients 2 to 3 months after transplantation. Cells were stained with fluorescently labeled antibodies that recognize the transferrin receptor (CD71) and the erythroid-specific antigen Ter119. Ter119⁺ erythroid cells were grouped into CD71 high (Region I), low (Region II), and negative (Region III) populations. The donor-derived erythroid cells in each region were determined by the percentage of GFP⁺ cells in the histograms to the right. Although donor-derived erythroid cells were only a small fraction of the early erythroblasts in Regions I and II, they represent the majority of late erythroblast and enucleated RBCs in Region III. (B) Low-donor chimerism in early erythroblasts (Regions I and II) and simultaneous high-donor chimerism in late erythroid cells (Region III) was consistent in all of the transplanted animals that were analyzed.