1. The rat reticulocytes examined contain 6.7 ± 0.2 mg. of phospholipid per ml. of packed cells, 49 per cent more than rat erythrocytes.
2. Reticulocytes from anemic animals appear to contain the same concentration of phospholipid as those from normal animals.
3. The concentration of total cholesterol in the reticulocytes examined does not differ significantly from that in erythrocytes.
4. The phospholipid:cholesterol ratio in lipid from reticulocytes is significantly greater (P < 0.001) than the corresponding ratio in lipid from erythrocytes. This finding, together with previous observations on the ultrastructure of reticulocytes, leads to the hypothesis that a substantial proportion of the lipid of these cells may be associated with phospholipid-rich, cholesterol-poor, intracellular structures such as mitochondria and endoplasmic reticulum.