Separation of bone marrow cells in continuous bovine serum albumin density gradients and the agar culture technique were used to determine the buoyant density distribution of granulocyte‐macrophage colony forming cells (CFUc) from normal marrow. The results showed that the byoyant density distribution of normal CFUc is dependent on the activity of the feeder layers used to stimulate colony formation and that additional low density CFUc grow on high activity feeder layers. The implications of these findings for studies of the density of acute meyloid leukaemia CFUc will be discusse