Complete or near complete remission of acquired pure red cell aplasia, not associated with thymoma, occurred in response to prednisone (six occasions), prednisone and cyclophosphamide (twice), cyclophosphamide alone (once) and splenectomy (once). These results were observed in four patients and confirm the value of cyclophosphamide and possibly splenectomy in patients resistant to prednisone in acquired red cell aplasia. In two of these patients, bone marrow progression from a state of ineffective erythropoiesis with many red cell precursors to that of red cell aplasia was observed. This indicates that some acquired refractory anaemias characterized by ineffective erythropoiesis may have the same pathogenesis as acquired red cell aplasia. The absence of demonstrable antibodies active against erythroblasts may merely reflect the insensitivity of currently available techniques. However it is possible that non-humoral immune mechanisms may be involved in the production of some forms of acquired red cell aplasia responsive to immunosuppressive drugs.