Red blood cells (RBC) from 13 patients suffering from dysmyelopoietic syndromes (DMPS) have been studied. About half of these subjects showed an abnormal Na+ and K+ leakage of their RBC, which were incubated at 37 °C for 24 h. The mean rate of the glycolytic pathway increased significantly and the glycolysis values correlated well with the Na+ gain. Moreover, 9 out of the 13 patients showed an abnormal biosynthetic ratio of haemoglobin chains in their reticulocytes. Since the multiple defects in DMPS erythrocytes do not seem clearly linked by cause-effect relationships, they probably derive from several cooperating factors in pathological erythroid precursors, leading to RBC membrane damage