A patient with severe AIHA of the warm antibody type, absence of reticulocytes and red cell hyperplasia of the bone marrow is described. In order to maintain a reasonable hemoglobin level 38 units of washed packed red cells were required within 24 days. The treatment with high doses of steroids showed no permanent beneficial effect. After splenectomy the red cell destruction was immediately reduced and the patient went into a remission. Bone marrow culture studies during the acute phase of the disease and at the time of complete hemato- and immunological remission, i.e. 4 months after splenectomy suggested a circulating autoantibody directed to early erythroid progenitors (BFU-E). The inhibitory activity in the patient's plasma did not influence granulocytic or mixed colony formation (CFU-GEMM). In addition to autoantibodies directed to erythroblasts and erythropoietin involved in the pathogenic mechanisms leading to red cell aplasia type I and II the culture studies suggest an unusual autoantibody that might cause the observed reticulocytopenia and erythropoietic hyperplasia of the bone marrow in AIHA. After the splenectomy the patient recovered, he required no further blood transfusions and his disease has not recurred.