CFU-E growth from fractionated bone marrow cells of 5 children with Diamond-Blackfan syndrome was studied. In all patients, CFU-E growth was reduced in mononuclear cell-rich fraction. In 2 of the 5 patients, CFU-E growth was returned to normal by the depletion of E-rosette forming cells or monocytes from mononuclear cell-rich fraction. In the patient whose CFU-E growth returned to normal by the depletion of E-rosette forming cells, co-cultivation between bone marrow buffy coat cells and autologous bone marrow E-rosette forming cells resulted in a significant decrease of CFU-E growth, and there was a significant increase in CFU-E growth by treatment with monoclonal antiserum to OKT4. We concluded that immunologic causes such as cellular factors may play a role, at least in part, in the pathogenesis of Diamond-Blackfan syndrome.