The deformability of red blood cells flowing in microvessels is essential to maintain optimal blood circulation and to allow gas transfer between blood and tissues. Here, we report on an experimental methodology to investigate the deformability of RBCs flowing in microcapillaries having diameter close to the average cell size. The microcapillaries are placed in a rectangular flow cell, where a suspension of RBCs, properly diluted in albumin-additioned ACD, is fed through a syringe under the action of a liquid head in the physiological range. Video microscopy images of the flowing RBCs are acquired at high magnification and later processed by an automated image analysis macro. It was found that RBCs from healthy donors exhibit the classical parachute shape observed in vivo. Furthermore, all the data of healthy RBC velocity vs liquid head are well represented by the same linear regression, independently on the donor. Preliminary results on beta-thalassemia RBCs are also presented and show, on the average, a reduced velocity compared to healthy samples.