A hemolytic disorder characterized by altered RBC cation composition (increases Na, decreases K), reduced monovalent cation content (decreased Na + K/liter RBC), and decreased levels of 2,3- diphosphoglycerate (2,3-DPG) is described. The etiology of these RBC abnormalities was not elucidated following extensive laboratory evaluation, although two important physiologic principles were manifested by this case: (1) Hemolysis was relatively well compensated (41% hematocrit) despite a significantly decreased RBC survival (51 Cr t 1/2 = 10.5 days). This effect presumably was due to reduced 2,3-DPG content (1.9 mumol/ml RBC) and the associated increase in whole blood oxygen affinity (P50 = 19.6 mm hg). (2) RBC size and water content were normal in spite of marked cation depletion. This anomaly was thought to reflect the osmotic effects of reduced polyvalent anion (2,3-DPG) content.