We examined the effect of magnesium (Mg) on cadmium (Cd) toxicity symptoms, uptake, and sorption to the plasma membrane in barley (Hordeum vulgare L. ‘Minorimugi’). Plants were grown in a constant concentration of Cd with variable Mg concentrations. Plants supplied with Cd only had necrotic first leaves, undeveloped emerging leaves, and outgrown lateral buds. The latter may account for greater dry mass accumulation in plants treated with Cd only than plants supplied with Mg. The symptoms of Cd stress were at least partly alleviated by the addition of Mg to the medium. Mg reduced accumulation of Cd in both shoots and roots, although there was a clear dose response in shoots only. We conclude that Mg inhibited Cd accumulation either by competing with Cd for uptake by the root or by inhibiting Cd translocation to the shoot. Cd sorption to plasma membrane vesicles from roots was unaffected by a concentration of Mg similar to that of Cd, but was reduced by high concentrations of Mg. This evidence of only weak competition between the two minerals suggests that the primary effect of Mg on reducing Cd accumulation is not through competition at the root surface but that Mg inhibits Cd translocation to shoots.