There is indirect evidence that the plasma membrane protein CFTR influences transepithelial transport by a change in cell elasticity. CFTR forms clusters with a variety of other proteins and enzymes. Some of these proteins interact with the cytoskeleton in a way that may change cell stiffness.Therefore, we developed a method to quantify membrane stiffness on living cells with atomic force microscopy. We quantified the membrane stiffness of native CHO cells, of wtCFTR transfected CHO cells (BQ2-CHO) and F508del CFTR transfected CHO cells (dF508- CHO) as well as endogenous CFTR expressing cell lines (Calu-3, 16HBE14o-). The membrane stiffness of native CHO cells and dF508-CHO cells is statistically not distinguishable, 0.96 ± 0.03 pN / nm (n = 90) and 1.0 ± 0.05 pN / nm (n = 116), respectively). The BQ2-CHO cells are 25% softer, showing a membrane stiffness of 0.75 ± 0.02 pN / nm (n = 88). Also Calu-3 (0.59 ±?0.032 pN/nm, n= 46) and 16HBE14o- (0.52 ± 0.051 pN/nm, n = 34) are soft. Stimulation with cAMP decreases the stiffness of wtCFTR expressing cells by ~25 % whereas the stiffness of dF508-CHO and CHO cells decreases by only 8%. From these data we conclude, that the presence and activation of wtCFTR in plasma membrane reduces cell stiffness. We hypothesize that the lack of CFTR prevents the formation of the CFTR multi-protein clusters and therefore stiffens the cell.