The cell-cycle properties of marrow CFUE were assessed in normal regenerating cell preparations. In addition, the erythropoietin sensitivity of CFUE was correlated to the cell-cycle distribution of these cells. The results suggest a coupling of erythropoietin sensitivity to the S-phase of the CFUE cell cycle in so far as CFUE in G2/M/G1 were 2 to 10 times less sensitive to the hormone. It is concluded that the CFUE cell cycle per se is not a focus for erythropoietic regulation.