1. 1. The influence of Na+ and K+ ions on the phosphate metabolism of the phosphoproteins of human red cells has been studied. 2. 2. With high intracellular Na+, but in the absence of external K+, the radioactivity of the phosphoproteins increases steadily over a period of at least 4 h. 3. 3. In the presence of external K+, or if external K+ is added during the incubation, the number of radioactive phosphate groups in the proteins declines and reaches a steady-state level at about 67% of that in the absence of external K+. 4. 4. Ouabain abolishes the effect of K+ ions, and also reduces the build up of radioactive phosphoprotein in the presence of intracellular Na+ ions. 5. 5. A protein phosphokinase has been demonstrated in purified red-cell membranes. It is activated by a number of monovalent cations. 6. 6. A phosphatase has been demonstrated to exist in these membranes. This enzyme splits p-nitrophenyl phosphate, is stimulated by K+ ions and inhibited by ouabain.