Committed erythroid progenitors are CD34⁺flt3^-, but CD34⁺flt3⁺ reconstituting HSCs have erythroid differentiation potential. To evaluate the frequency of BFU-E, sorted CD34⁺flt3⁺(▪) and CD34⁺flt3^-(□) cells from CB and adult BM were plated directly after sorting in methylcellulose cultures containing SCF⁺EPO. To measure the frequency of erythroid progenitors generated during long-term culture, sorted CD34⁺flt3⁺and CD34⁺flt3^-cells from CB and adult BM were cocultured on stroma layers for 6 weeks and then were plated in methylcellulose cultures containing SCF⁺EPO (LTC–BFU-E). Finally, to evaluate the frequency of erythroid progenitors generated from SRC, sorted CD34⁺flt3⁺and CD34⁺flt3^-cells from CB were transplanted into NOD/SCID mice; 6 to 8 weeks after transplantation, unfractionated BM cells harvested from mice were plated in methylcellulose cultures containing SCF⁺EPO (SRC–BFU-E). BFU-Es were scored after 14 days. Results are expressed as number of erythroid colonies per 500 CD34⁺flt3⁺and CD34⁺flt3^-cells for BFU-E and LTC–BFU-E and per 1 × 10⁶ total NOD/SCID BM cells for SRC-BFU-E. Data represent mean values from 3 experiments using cord blood and 2 experiments using adult bone marrow cells with 3 to 6 replicates in each group; error bars depict SEMs.