  Polariton lasing is demonstrated on the zero dimensional states of single
GaAs/GaAlAs micropillar cavities. Under non resonant excitation, the measured
polariton ground state occupancy is found to be as large as $10^{4}$. Changing
the spatial excitation conditions, competition between several polariton lasing
modes is observed, ruling out Bose-Einstein condensation. When the polariton
state occupancy increases, the emission blueshift is the signature of
self-interaction within the half-light half-matter polariton lasing mode.
