FOUR FIGURESEnzymes that liberate orthophosphate from adenosine triphosphate are widely distributed in animal and plant cells, but comparatively little is known about their exact physiological function. The present paper had its origin in the finding of Rothstein and Meier ( '48) that dephospliorylation of ATP by intact yeast cells occurs at the cell surface. It was thought that if a similar condition exists in other cells, a particularly favorable form of material for its study might be the mammalian erythrocyte, which is unique in the ease and completeness with which its surface ("ghost," stroma) can be separated from its other constituents. This surmise proved to be correct.As the work was nearing its completion, some of its conclusions were anticipated by a paper by Clarkson and Maizels ( '52). I n general, however, the points of view of the two investigations are entirely different and the present results for the most part do not duplicate those already published. I n the