A component of active sodium efflux has been described by Hoffman and Kregenow (1966, Ann. N.Y. Acad. Sci. 137, 566–567) which is not inhibited by ouabain but is inhibited by ethacrynic acid. This has been called "Pump II" and these authors have produced evidence suggesting that the process does not require adenosine triphosphate (ATP) for its energy supply. Studies of membrane adenosine triphosphatase (ATPase) from human erythrocytes have therefore been carried out to determine if any component of this enzyme behaved in a qualitatively similar way to that of "Pump II." Our results have shown that ethacrynic acid behaved like a nonspecific sulfhydryl-reacting substance with no discrete inhibitory properties. Two analogues of ethacrynic acid, which did not have any sulfhydryl-reacting properties, could also inhibit "Pump II" but did not inhibit membrane ATPase.It is concluded that there is no direct qualitative correlation between "Pump II" and membrane ATPase in the human red cell. This supports the view that "Pump II" functions independently of the activity of membrane ATPase. Whether "Pump II" is an active transport system or part of a cation exchange diffusion mechanism, it is unlikely that ATP or membrane ATPase are concerned in its function.