CFU-E colony frequencies were determined in methylcellulose assays for total BM cells, for sorted EPs, and for BM cells depleted of EPs by cell sorting. CFU-E colony numbers were measured per 10⁵ total BM cells and per 10⁵ BM cells depleted of EPs. For EPs, CFU-E colony numbers were calculated per 10⁵ cells. We obtained 648 CFU-E colonies per 850 EPs plated (experiment 1), 345 CFU-E colonies per 500 EPs plated (experiment 2), and 405 CFU-E colonies per 500 EPs plated (experiment 3). Thus, EPs gave rise to CFU-E colonies with about 70% efficiency. An approximate 8-fold depletion of EPs from BM reduced the CFU-E frequencies in BM by about 10-fold (see “Materials and methods”). Hence, the vast majority of CFU-E activity in BM was quantitatively isolated with the EP phenotype.