A study of the osmotic resistance of red cells from normal subjects (AA), patients with the sickle cell trait (heterozygous subjects) (AS) and homozygous subjects (SS) was carried out using a dynamic automatic micro-method: Danon's fragiligraph, permitting one to obtain simultaneous recording of cumulative and derivative curves of the hemolytic process. 15 samples of red cells of each of the three categories were studied. Fragiligrams obtained from SS red cells showed that they became hemolysed later and in a more progressive fashion than the normal red cells AA. The AS red cells occupied an intermediate position. 5 samples of each category of red cells were studied after various times of incubation in the presence of a reducing substance (sodium metabisulphite). It was shown that the red cells containing hemoglobin S were still more resistant to osmotic hemolysis when hemoglobin was in the deoxygenated form and this proportional to the quantity of deoxygenated hemoglobin, without the brutal process of sickling producing any special modification in the shape of the curve. The role of the membrane is considered.