The ouabain resistant and ouabain-sensitive alpha -subunit cRNAs in various molar ratios were injected into Xenopus oocytes together with cRNA for the beta -subunit. The ouabain-resistant ATPase activity, as well as ouabain-resistant Rb+ uptake, of the injected oocytes increased linearly with increasing the amount of cRNA for the ouabain-resistant alpha -subunit. When a functionless mutant was used instead of the ouabain-sensitive alpha -subunit, similar results were obtained in ATPase activity and Rb+ uptake. These results indicate that a monomeric alpha beta protomer is a functional unit of membrane-bound Na,K-ATPase, even if the enzyme exists structurally as a diprotomer or higher oligomers in membranes.