Résumé : Lateral migration of vesicles (closed lipidic membranes) in a flow is characterized as a function of the relevant flow parameters and mechanical properties of the vesicles. We consider low Reynolds number flows, and migration is only due to viscous effects. Through experiments and simulations, we exhibit two different origins for such cross streamline migration: the presence of a wall, and a non-constant shear rate, as for instance in a Poiseuille flow. Such migration modifies the distribution of vesicles in a sheared polydisperse suspension; we present preliminary results proving that the hydrodynamic interactions between vesicles greatly modify the distribution of vesicles according to their sizes and deflation. © 2009 Materials Research Society.