The sedimentation velocity profiles of the entities in mouse bone marrow responsible for erythropoietic burst formation (BFU-E) and for erythrocytic colony formation (CFU-E) were studied under conditions designed to determine whether the values observed were real or resulted from cell interactions occurring during culture of the fractions. Bone marrow cells of adult C3Hf/Bi mice were subjected to unit gravity sedimentation in a bovine serum albumin gradient, and fractions were assayed in plasma culture. Because it was found that cell concentration affected the efficiency of erythropoietic burst formation in culture, aliquots were plated at 2 different cell concentrations, as well as at a fixed proportion of each fraction. The modal sedimentation velocity of the BFU-E population averaged 3.9 mm/h and that if the CFU-E population, 6.4 mm/h; both were independent of cell concentration or method of dividing the fractions. Cells from fractions of different sedimentation velocity were mixed with one another or with unfractionated cells. No significant inhibition or stimulation of erythropoietic burst formation was seen. The observed values apparently represented the true modal sedimentation velocities of BFU-E and CFU-E in normal mice. To determine whether a change in the physiologic state of the animals affected the sedimentation velocities of BFU-E or CFU-E, marrow cells from mice hypertransfused with red cells were compared with those from controls. The model sedimentation velocity of BFU-E was unaffected by hypertransfusion, nor was there any change in the number of BFU-E under these conditions. The number of CFU-E was substantially reduced without a significant change in modal sedimentation velocity.