Pure erythrocyte aplasia is a recognised feature of systemic lupus erythematosus (SLE); here we report two cases, one predating the onset of SLE, the other following a long period of disease quiescence. One case demonstrates the typical features of this disorder and was successfully treated with prednisolone. The second case is unusual in being resistant to immunosuppressive treatment. Bone marrow culture from the second patient revealed an inhibition of BFU-E colony formation in the presence of the patient's serum, indicating that a serum inhibitor of haemopoiesis was present. Furthermore, following T cell depletion of this patient's marrow, there was an increase in BFU-E, CFU-G and CFU-GM colony growth implicating, in addition, a possible T cell-mediated inhibition of marrow haemopoiesis. This is a novel observation and may explain the resistance shown by this patient to standard treatment.