Potassium (K+) influx and 3H-ouabain binding were studied in human red cells completely lacking the rhesus (Rh) antigens (Rhnull cells) and compared with normal Rh(D) red cells. The Rhnull cells, originally described by Seidl, Spielmann, and Martin (Vox Sang. 23:182, 1972) were normal in size, cation, and water content, indicating no significant increase in cell volume as occurs in young human red cells. However, the ouabain-insensitive K+ permeability, as well as the ouabain sensitive active K+ transport, were increased 1.6 1.8-and 1.4-1.5-fold, respectively, above the values found in Rh(D) control cells. The Na+K+ ATPase activity of membranes from Rhnull cells was also higher than from Rh(D) cells. Binding studies with 3H-ouabain revealed that at 100% K+ pump inhibition Rhnull cells bound 670 and Rh(D) cells 450-500 ouabain molecules per cell. Since the rate of ouabain binding was identical in Rhnull and Rh(D) control cells, we concluded that the Rhnull cell had about 35%-45% more cation pumps than the Rh(D) cell. These additional pumps in Rhnull cells appeared to be indistinguishable from those in control cells. Anti-D or the serum from the Rhnull individual did not alter cation permeability in Rh(D) red cells. The data suggested that the Rhnull cell, known for its hematologic malfunction, was not a young or prematurely released red cell, but had a pleiotropic membrane defect which also affected the passive and active cation transport system on the molecular level. Our finding precludes a structural identity of the rhesus antigen with the molecules composing the Na+K+ pump system.