We present the quantum theory of momentum and spin relaxation of exciton-polaritons in microcavities. We show that giant longitudinal-transverse splitting of the polaritons mixes their spin states, which results in beats between right- and left-circularly polarized photoluminescence of microcavities, as was recently experimentally observed [M.D. Martin, Phys. Rev. Lett. 89, 077402 (2002)]. This effect is strongly sensitive to the bosonic stimulation of polariton scattering.