Sickle cell disease (SCD) is the most frequent genetic disease in the West Indies and in France. This disorder is caused by polymerization of the abnormal hemoglobin S which results from the substitution of an acid glutamic by a valine at codon 6 of the beta globin chain. Polymerization of deoxygenated hemoglobin S induces formation of long stiff rod-like fibers which force the red blood cells to take over a wide variety of irregular shape. SCD patients exhibit life-threatening complications such as chronic anemia and vaso-occlusion events due to the loss of red blood cell (RBC) deformability and increase of RBC adherence to endothelial cells [1]. In this study, the mechanical properties of the red blood cells are investigated by atomic force microscopy. For this purpose, the cells are immobilized on glass lamella coated with poly-l-lisine in order to increase RBC adherence. All the experiments are performed in presence of phosphate buffered saline (PBS). Red blood cells were imaged with silicon nitride probe with nominal spring constant of 0.06 N/m whereas the mechanical measurements were performed with silicon tips colloidal particle with nominal spring constant of 0.035 N/m. More than 150 mechanical tests have been performed on each sample in order to have reliable statistical data. All force/deformation curves were analyzed with PUNIAS (Protein Unfolding and Nano-Indentation Analysis Software), a custom-built semi-automatic processing and analysis software. To calculate the young modulus, we used Sneddon’s modification of the Hertz model for the elastic indentation of a flat and soft sample by a stiff sphere [2]. Four different groups of patient were analyzed, AA, AS, SS and SS with hydroxyurea treatment. The results show that the SS red blood cells are stiffer than AA cells, whereas we do not observed any difference between AS and SS cells. We also observed a decrease of the young modulus for the patients treated with hydroxyurea. REFERENCES [1] Microelasticity of red blood cells in sickle cell disease, J. L. Maciaszek, B. Andemariam, and G Lykotrafitis, J. Strain Anal. Eng. Des. 46(5), 368–379 (2011) [2] Elasticity measurement of living cells with an atomic force microscope: data acquisition and processing, Carl P. and Schillers H., Pflugers Arch - Eur J Physiol, 457:551–559 (2008)