A useful way of obtaining more accurate information about the processes that occur when fast plasmoids are trapped in magnetic traps is to investigate the interaction of the fast plasmoids with the barriers of magnetic fields that are constant or variable in time. The results obtained may be valuable in helping to clarify questions concerning the acceleration of plasmoids by variable magnetic fields and to explain some of the processes involved in penetration of the magnetic field by the plasma. To investigate the effects of the interaction between fast plasmoids and magnetic field barriers a coaxial plasma gun of the Marshall type was constructed. When the optimum conditions for operating the gun had been established plasmoids with a speed of 1.3 × 107 cm/s and an initial density of 2.5 × 1013 ion pairs per cma were obtained. During investigation of plasmoid interaction with the barrier of a constant magnetic field, compression of the plasma was observed at low field intensities. At high magnetic field intensities the density of the plasmoids penetrating the barrier decreased until the plasma was completely reflected by the barrier. Using the model of a collisionless plasma, equations were obtained for the distribution of electromagnetic fields along the radius of a plasmoid passing through the barrier. By comparing the solution of this equation with the measured diamagnetic signal the electron gas density in the plasmoid was determined; it was found to be in agreement with results obtained with a twin probe. The interaction of plasmoids with the barrier of a variable magnetic field is determined not only by the amplitude of the field intensity, but also by a whole set of interaction phases. New values characterizing these phases were introduced for convenience. The reflection, partial penetration and acceleration of the plasmoids was measured for different interaction parameters.