W e investigated the erythrocyte membrane proteins of two patients with congenital hemolytic anemia due to increased permeability of the erythrocyte membrane to Na and K (hereditary stomatocytosis and cryohydrocytosis). One-dimensional sodium dodecyl sulfate (SDS) gel electrophoresis resolved the band 7 erythrocyte membrane proteins into three components with approximate molecular weights of 30,000, 28,000, and 26,000. The 28,000-dalton component was decreased in both patients with permeability disorders. Two-dimensional electrophoresis (nonequilibrium pH gradient electrophoresis in the first dimension combined with SDS gel electrophoresis in the first dimension combined with SDS gel electrophoresis in the second dimension) resolved the 28,000-dalton component from normal erythrocyte membranes into two proteins with different isoelectric points, designated 22 x 8 and 60 x 8. In the patients with hereditary stomatocytosis and cryohydrocytosis, 22 x 8 was completely absent, whereas 60 x 8 was detected as usual. In contrast, all the band 7 proteins (including 22 x 8) were invariably present in a survey of normal subjects and reticulocytosis controls. The unique finding of a missing band 7 protein in the patients with hereditary stomatocytosis and cryohydrocytosis raises the possibility that the absence of this protein is responsible for the increased Na and K permeability in these disorders.