With use of the positively charged, colloidal ferric oxide labelling technique for electron microscopy of sections of rabbit marrow, a reduction in labelling density on the surface of differentiating red cell precursors was demonstrated. The number of iron particles per unit length of membrane was counted. A progressive diminution in labelling density follows cell division, reaching a minimum in the orthochromatic erythroblast, from which the nucleus is expelled. A slight increase in charge density is noted in the reticulocyte, and further increase is observed with its maturation to the erythrocyte. The results indicate that biosynthesis of n-acetyl neuraminic acid stops at the earliest recognizable stage of erythroid differentiation.