  We present novel experimental results demonstrating the coherence properties
of the nonlinear emission from semiconductor microcavities in the strong
coupling regime, recently interpreted by parametric polariton four-wave mixing.
We use a geometry corresponding to degenerate four-wave mixing. In addition to
the predicted threshold dependence of the emission on the pump power and
spectral blue shift, we observe a phase dependence of the amplification which
is a signature of a coherent polariton wave mixing process.
