T cells stimulate the proliferation of BFUE (burst forming units-erythroid) from normal blood null cells in an in vitro culture system in the presence of erythropoietin. To determine whether abnormal BFUE proliferating effect of T cells could explain the pure red cell aplasia in chronic lymphocytic leukemia (CLL-PRCA), we investigated the erythropoietic function of T and null cells in four patients with CLL-PRCA and compared results to three patients with idiopathic pure red cell aplasia (I-PRCA) and normals. Sera from I-PRCA patients (P greater than 0.05) but not CLL-PRCA patients (P less than 0.1) inhibited erythroid stem cell proliferation in the presence of complement. BFUE in null cells of all PRCA patients were barely detectable or absent (P less than 0.0025). Normal or I-PRCA T cells increased BFUE proliferation from PRCA null cells of six patients (P less than 0.001). In contrast, CLL-PRCA T cells were poor stimulators of BFUE from autologous (P less than 0.001) or allogeneic null cells (P less than 0.02). Treatment with cyclophosphamide and prednisone induced reticulocytosis in all four CLL-PRCA patients. After treatment, in two cases, the burst promoting function of T lymphocytes was normal. Analysis of T cell subpopulations in two CLL-PRCA patients, suggested that the reduced burst promoting function was due to decreased numbers and/or function of T cells bearing Fc receptors for IgM (TM cells). These findings suggest that reduced generation of a burst promoting activity by CLL-PRCA T cells may contribute to the pathogenesis of PRCA in chronic lymphocytic leukemia.