An influence of a constant magnetic field (MF) with induction up to 15 T upon macroscopic plastic deformation of diamagnetic NaCl single crystals containing paramagnetic Eu impurity was observed. Effects of MF on instability of plastic flow (on the Portevin-Le Chatelier effect) in quenched NaCl:Eu crystals were found. The MF results in a decrease of the yield stress, a decrease of the probability of the instability appearance, and a decrease of the averaged magnitude of the deformation jumps, as well as in a chaotization of the distribution of the deformation jump magnitudes. The latter can be explained by partial plastic relaxation of internal stresses under MF that was confirmed by dislocations displacement induced by internal stresses in MF. The averaged amount of shear bands observed on the surface of the investigated crystals deformed under MF is half of that in reference specimens.