The incubation of ghosts derived from human Rhesus-positive red cells with IgG-anti-Rhesus-D inhibited the K+-sensitive p-nitrophenylphosphatase activity. This enzyme has a partial function in the (Na+ + K+)-ATPase system related to the phosphorylation step, which is important for active potassium transport through the red cell membrane. The specificity of the impairment by the antigen-antibody reaction in the Rhesus-D system was proved by the following controls. Ghosts obtained from Rhesus-negative red cells incubated by IgG-anti-Rhesus-D and those of Rhesus-positive red cells treated with non-immune serum did not show any reduction of the K+-p-nitrophenylphosphatase activity. The ghost preparation with lanthanum carried out after hypotonic hemolysis of the washed red cells in 2 mM LaCl3 at pH 6 was the most suitable procedure to explore this topic in comparison to other techniques for preparing ghosts of red cells.