THE pluripotent haemopoietic stem cell has been the object of many investigations in murine and human systems, but the origin of lymphoid cells from the human pluripotent stem cell has not been definitively established. In mice, investigations using the spleen colony technique1 have demonstrated the existence of a pluripotent stem cell which can give rise to mixed colonies of erythrocytes and granulocytes2. Further studies with radiation-induced chromosome markers in murine systems have provided evidence for the origin of lymphoid cells from this stem cell3-5. The use of X-linked glucose-6-phosphate dehydrogenase (G-6-PD) isoenzymes as cell markers has provided another approach, applicable to the evaluation of human proliferative disorders6,7. Those females with electrophoretically distinguishable G-6-PD isoenzymes who have clonal haematological disorders provide a source for such studies. Thus, Fialkow et al.8 have demonstrated the origin of erythrocytes, granulocytes, megakaryocytes, and macrophages from a myelogenous stem cell in studies of patients with chronic myelocytic leukaemia. We have used G-6-PD markers in a study of peripheral blood cells from a patient with acquired idiopathic sideroblastic anaemia, and shown the origin in humans of B and T lymphocytes from a common stem cell whose other progeny include erythrocytes, granulocytes, megakaryocytes, and macrophages. In addition, acquired idiopathic sideroblastic anaemia is shown, in this instance, to be a clonal disorder. © 1978 Nature Publishing Group.