The degree of circular polarization () of the nonlinear emission in semiconductor microcavities is controlled by changing the exciton-cavity detuning. The polariton relaxation towards cavitylike states is governed by final-state stimulated scattering. The helicity of the emission is selected due to the lifting of the degeneracy of the spin levels at . At short times after a pulsed excitation reaches very large values, either positive or negative, as a result of stimulated scattering to the spin level of lowest energy (<?format ?>+<?format ?>1/<?format ?>-<?format ?>1 spin for positive/negative detuning).