  The degree of circular polarization ($\wp$) of the non-linear emission in
semiconductor microcavities is controlled by changing the exciton-cavity
detuning. The polariton relaxation towards \textbf{K} $\sim 0$ cavity-like
states is governed by final-state stimulated scattering. The helicity of the
emission is selected due to the lifting of the degeneracy of the $\pm 1$ spin
levels at \textbf{K} $\sim 0$. At short times after a pulsed excitation $\wp$
reaches very large values, either positive or negative, as a result of
stimulated scattering to the spin level of lowest energy ($+1/-1$ spin for
positive/negative detuning).
