Plasmas with variable ratios of hot electrons to colder background plasma, but without significant neutral gas, are investigated in the multiple mirror experiment (MMX) [Phys. Fluids29, 1208 (1986)]. The device permits, sequentially, the injection of plasma (H2or He) into a stable mirror cell, and electron‐cyclotron resonance heating (ECRH) to an electron temperature of several keV. A second source then provides the cold‐electron component with arbitrarync/nh≤50. At any point in time, the field coils that provide magnetohydrodynamic (MHD) stability for the hot electrons may be pulsed off for times short in comparison to the time required for cold plasma density to change. Instantaneous destabilization, even for plasma parameters well in excess of the value ofnc/nhfor which a hot‐electron drift approximation would predict stability, was observed. This result suggests that a mechanism in addition to a cold plasma background is required to stabilize a hot‐electron distribution. Comparison is made with recent theoretical work.