1. Data have been presented to show that most reticulocytes from patients with the sickle cell trait or sickle cell anemia sickle as readily as do more mature red blood cells. The most immature reticulocytes and normoblasts tend to sickle more slowly.
2. Orthochromatic normoblasts were the only type of normoblasts which sickled; the basophilic and polychromatophilic types could not be sickled.
3. It is suggested that the sickle cell forms seen in ordinary stained smears represent old cells which have lost their "elasticity" while stagnating in the sickle shape, and are unable to revert to a biconcave disk. This would explain the fact that these forms are so rarely found to be reticulated when stained with brilliant cresyl blue.
ACKNOWLEDGMENT I wish to express my gratitude to Dr. William B. Castle, Dr. William Dock (Department of Medicine) for many stimulating suggestions, to Dr. John M. Pearce (Department of Pathology) for assistance in preparing this report and to Mrs. Muriel MacDowell (Department of Pathology) for the photomicrography.