The polarization of semiconductor microcavities with embedded quantum wells (QWs) in the regime of polariton lasing is studied theoretically. We expect the emitted light to be linearly polarized, with the degree of polarization as well as its direction determined by the asymmetry of QW interfaces or localization of excitons on the anisotropic islands of QW width fluctuations. The ways to control the direction of polarization using external electric field are analyzed.