IN previous work1 we have shown that liver slices suspended in a medium free of sodium ions show a depressed turn-over of phosphoprotein phosphorus. This effect is reversed by addition of sodium. In these and the experiments shown here, the phosphoproteins have been identified by isolation of radioactive pbosphorylserine from acid hydrolysates of the crude proteins. We have found that the phosphoproteins sensitive to Na+ are non-mitochondrial1, the mitochondrial phosphoproteins appearing to have different properties2.