The pathogenesis of the anemia associated with malignancy was investigated in a patient with T cell chronic lymphocytic leukemia. The plasma clot culture system was used as a measure in vitro of erythropoiesis. The patient's peripheral blood and marrow T lymphocytes obtained both before and after transfusion therapy suppressed erythroid colony formation by normal human bone marrow cells. Pretreatment of the patient's bone marrow T cells by antithymocyte globulin (ATG) and complement reversed this suppression. In addition, pretreatment of the patient's marrow cells with ATG and complement markedly augmented erythropoiesis in vitro. The expression of erythroid activity caused by the selective destruction of the suppressor T lymphocytes in the patient's bone marrow with ATG and the suppression of normal erythropoiesis by the patient's bone marrow and peripheral blood lymphocytes suggest that interaction between the malignant T cell and the erythropoietin-responsive stem cell is important in production of anemia in this patient.