A high &bgr; plasma is injected into an axisymmetric magnetic mirror. The &bgr; is varied principally by varying the midplane value of the magnetic field. The plasma fills most of the space inside of a conducting vacuum chamber. It is observed for a radially averaged ⟨&bgr;⟩>35% that the plasma is initially stable, while for ⟨&bgr;⟩<25% the plasma is initially unstable. The values of &bgr; required for stability are lower than those predicted from magnetohydrodynamic (MHD) theory.