Femoral bone marrow was divided longitudinally into two groups of cells of varying size. By assaying CFU and CFU in the two zones of the marrow, their distributions across the diameter of the femur was determined. It is shown that the concentration of CFU increases from the femoral axis (15 CFU/105 bone marrow cells) to the bone surface (44 CFU/105 cells), obeying approximately a square-law relationship. The CFU concentration, on the other hand, increases from the femoral axis (32CFU/105 cells) to a peak value (260 CFU/105 cells) at about 330 um from the axis and thence falls off against to the bone surface (77 CFU/105 cells). Selective kinning cells in DNA synthesis using the tritiated thymidine suicide technique, in vivo, showed that CFU, near the bone surface are proliferating at a faster rate than those more distant from bone, but that CFU have a fast proliferation rate irrespective of their position in the distribution. Thus, bone marrow cell populations are shown to conform to a well-defined spatial organization corresponding to the chronologic relationships between marrow cells.