The great current interest in plasma fats and their relation to disorders of obvious clinical importance (e. g., arteriosclerosis) has tended to obscure in some degree certain other recent developments in knowledge of lipid metabolism. Among these, it is possible to record for the lipids of the red blood cell advances in our knowledge of their composition as well as a beginning of understanding of the dynamics of their metabolism. While it is too soon to appraise the importance of such researches for medicine, they appear to bear upon at least two old and interrelated problems of physiology. The first of these concerns the architecture of the cell, especially its membranes. It has long been supposed that phospholipids have important structural functions, including those concerned with maintaining the characteristic cellular permeability. The second problem is that of metabolic activity, changes in which may be associated, for example, with hemolytic disease