We present the semi-classical theory of the non-linear propagation of polarized light in semiconductor microcavities. This formalism explains the mysterious rotation of the polarization plane of light emitted by a microcavity in the regime of stimulated scattering observed recently. The model describes the stimulated four-wave mixing in microcavities. We show that the exciton spin-splitting induced by the polarized pumping is responsible for giant resonant Faraday rotation of the polarization plane of light emitted by the cavity.