Pure red cell aplasia developed in a female patient with systemic lupus erythematosus (SLE). Erythroid colony growth was assessed in semisolid medium culture of bone marrow obtained from a normal donor and cultured in the presence of normal and patient sera. Colony forming units of erythropoiesis and burst forming units of erythropoiesis obtained from a normal donor were inhibited in the presence of patient sera. Our findings support the concept that circulating inhibitors might influence the proliferation of erythroid progenitor cells and erythroid aplasia may be an immunologically mediated syndrome.