Procedures have been developed to label human erythropoietin (Ep) with biotin to detect and isolate the Ep-receptor. The labeling method used the abundant carbohydrate groups on Ep and resulted in biologically active biotin-Ep (b-Ep) containing 8 to 10 biotins per Ep molecule. Specific binding of b-Ep to cells from spleens of mice made anemic by phenylhydrazine injections was demonstrated using 125I-labeled streptavidin. B-Ep, together with fluorescently tagged streptavidin, was found to specifically detect Ep-receptor-bearing cells by flow cytometry. This was demonstrated in several ways. First, approximately 90% of nucleated spleen cells from phenylhydrazine-treated mice were clearly fluorescent after staining with b-Ep and streptavidin- phycoerythrin, whereas only background fluorescence was detected using spleen cells from untreated mice. In addition, Ep-receptors were detected on 5% to 10% of normal mouse bone marrow cells, and these cells could be identified as erythroid in nature by separating the cells into subpopulations based on light-scatter properties. Third, Ep- receptor expression was found to correlate positively with expression of transferrin receptors, confirming the erythroid nature of these cells. B-Ep was also used to isolate Ep-receptors from monkey COS cells transfected with the murine Ep-receptor cDNA. In these experiments a cell-surface-bound protein of approximately 65 Kd and an intracellular protein of approximately 60 Kd were isolated from these cells. The procedures described in this report for detecting Ep-receptor expressing cells and for isolating the Ep-receptor should be valuable for purifying erythroid cells from heterogeneous cell populations, for elucidating the structure of the Ep-receptor, and for studying the biological activities of Ep at the cellular and molecular level.