  We observe signatures of stimulated bosonic scattering of excitons, a
precursor of Bose-Einstein-Condensation (BEC), in the photoluminescence of
semiconductor quantum wells. The optical decay of a spinless molecule of two
excitons (biexciton) into an exciton and a photon with opposite angular momenta
is subject to bosonic enhancement in the presence of other excitons. In a spin
polarized gas of excitons the bosonic enhancement breaks the symmetry of two
equivalent decay channels leading to circularly polarized luminescence of the
biexciton with the sign opposite to the excitonic luminescence. Comparison of
experiment and many body theory proves stimulated scattering of excitons, but
excludes the presence of a fully condensed BEC-like state.
