Two-photon absorption spectra of the biexciton transition in CuCl have been investigated using a pump-probe technique. Combinations of two circularly-polarized lightbeams with various photon energies are used to examine the relation of the absorption to the transition selection rule. The absorption is found to occur even for the forbidden configuration of polarizations of two beams when the photon energy of the one incident light is close to the exciton resonance. The departure from the selection rule is shown to arise from depolarization of polaritons. The degree of depolarization depends on the polariton density. It is concluded that the elastic scattering between polaritons is the dominant process mediating the two-photon transition under forbidden configuration.