Summary. Two patients with chronic granulocytic leukaemia developed red cell aplasia during the course of their disease. In one of them, cell culture studies demonstrated the presence in the patient's serum of an IgG inhibitor of haemoglobin synthesis by his own mature erythroblasts and erythroblasts grown in vitro from his erythroid colony forming cells. The IgG fraction was also found to be cytotoxic for the patients’ marrow erythroblasts that were present after disappearance of the red cell aplasia. Treatment with corticosteroids resulted in reappearance of the erythroblasts in the marrow and decrease in the transfusion requirement. Red cell aplasia can occur before, at the same time or after the onset of chronic granulocytic leukaemia and may have the same immune pathogenesis as chronic idiopathic pure red cell aplasia. It occurs without busulphan treatment and seems to have no direct relation to the terminal metamorphosis. Treatment of the red cell aplasia with corticosteroids would appear worthwhile as it may reduce the transfusion requirement without affecting the course of the underlying leukaemia.

Two patients with chronic granulocytic leukaemia developed red cell aplasia during the course of their disease. In one of them, cell culture studies demonstrated the presence in the patient's serum of an IgG inhibitor of haemoglobin synthesis by his own mature erythroblasts and erythroblasts grown in vitro from his erythroid colony forming cells. The IgG fraction was also found to be cytotoxic for the patients' marrow erythroblasts that were present after disappearance of the red cell aplasia. Treatment with corticosteroids resulted in reappearance of the erythroblasts in the marrow and decrease in the transfusion requirement. Red cell aplasia can occur before, at the same time or after the onset of chronic granulocytic leukaemia and may have the same immune pathogenesis as chronic idiopathic pure red cell aplasia. It occurs without busulphan treatment and seems to have no direct relation to the terminal metamorphosis. Treatment of the red cell aplasia with corticosteroids would appear worthwhile as it may reduce the transfusion requirement without affecting the course of the underlying leukaemia.