  We report an experimental observation of polaritonic optical bistability of
the laser emission in a planar semiconductor microcavity with a $100\AA$ GaAs
single quantum well in the strong-coupling regime. The bistability curves show
crossings that indicate a competition between a Kerr-like effect induced by the
polariton population and thermal effects. Associated with the bistability,
laser-like emission occurs at the bare cavity mode.
