- The paper reexamines the question of efficiency of acceleration by magnetic magnitude fluctuations (mirrors) in the context of the various situations in which it has been invoked. The study points out that although a thermal particle absorbs less power than a relativistic one, the thermal particle will invariably be heated beyond observed limits before appreciable acceleration of suprathermal particles takes place. In this sense, acceleration by magnetic mirrors is exceedingly inefficient. A burst of large-scale turbulence, suitable for Fermi acceleration, would be degraded into some other form before accelerating any fast particles appreciably. It is claimed that moving magnitude fluctuations in the magnetic field are generally not a viable mechanism for fast particle acceleration.