Observations were made on the changes in volume, osmotic fragility, and cation contents of red cells incubated in serum at 37 C. for 24 and 48 hours. The results show that spontaneous autohemolysis is not due to progressive swelling of the cells, but is probably due to degenerative changes in the cell membranes. On incubation, normal red cells increase in volume during the first 24 hours due to a gain in sodium and water; the cells lose potassium but at a slower rate than they gain sodium. During the second 24 hours of incubation the loss in potassium exceeds the gain in sodium and the cells shrink to near their original volume. These cation changes and the autohemolysis are greatly reduced if glucose is present throughout the 48 hours of incubation. Red cells from several different types of congenital hemolytic anemia were also studied; important deviations from the normal pattern were observed. In hereditary spherocytosis the rates of autohemolysis, of increase in osmotic fragility, and of potassium loss are greater than normal. The continued presence of glucose during incubation markedly retarded these changes. In hereditary elliptocytosis trait the red cells behaved normally on incubation. In one case of elliptocytosis with hemolytic anemia, autohemolysis was normal but there was an increased potassium loss. In another patient with hemolytic anemia and increased osmotic fragility autohemolysis was greatly increased. In all these cases of elliptocytosis, glucose reduced the autohemolysis moderately but not to a normal degree. Four cases of congenital nonspherocytic hemolytic anemia were studied. In two patients (type 1) autohemolysis, osmotic fragility and cation changes on incubation were normal; glucose had a normal effect on the fragility and cation changes, but only slightly reduced the autohemolysis. In the two other patients (type 2) autohemolysis, increase in osmotic fragility, and loss of potassium were markedly increased. Glucose did not retard any of these changes and it was found that the cells were unable to utilize glucose at the normal rate.