The measurement of the osmotic fragility of red blood cells of 7 patients with sickle cell anemia and 5 controls was carried out for the unfractionated blood and for cells of the top and bottom layers after centrifugation. The erythrocytes from all patients with sickle cell anemia were more resistant to the osmotic lysis than normal erythrocytes, and the resistance of the bottom layer cells was greater than that of the top layer cells for all cases of sickle cell anemia. In addition, in the cases of sickle cell anemia, the bottom layer contained an elevated percentage (5.0% - 23.7%) of irreversible sickled cells, as compared to 1.3% - 3.6% in the top layer. The decreased osmotic fragility of the erythrocytes in sickle cell anemia probably results from the cell damage which accompanies repeated sickle-unsickle cycles and which reaches its maximum in the irreversibly sickled cells.