Early and late erythroblasts can be identified and isolated by flow cytometry. Immunophenotyping of marrow mononuclear cells with glycophorin A-PE and CD71-FITC can identify immature from mature erythroblasts, as confirmed by histochemical staining of the sorted populations (inset photographs). Samples were analyzed using an Olympus BX41 microscope equipped with a UPlanFL 40×/0.75 objective lens (Olympus, Melville, NY). Pictures were taken with Olympus QColor3, and were analyzed with QCapture 2.60 software (QImaging, Burnaby, BC, Canada) and Adobe Photoshop 6.0 software (Adobe, San Jose, CA).