Water containing high concentrations of magnesium ions (e.g. Dead Sea water) is effective in the treatment of inflammatory skin diseases. Therefore, we examined the influence of Mg2+ on inflammation in allergic contact dermatitis induced by 1-chloro-2,4-dinitrobenzene (DNCB) in BALB/c mice. Animals challenged with 0.125% DNCB in the presence of magnesium chloride (28% and 14%) demonstrated significantly less pronounced contact dermatitis (ear swelling) than did animals challenged with DNCB alone (p less than 0.001 and p less than 0.01). In mice challenged with DNCB in combination with sodium chloride (14%) there was no statistically significant difference in the degree of ear swelling. These results were borne out in 5 patients known to be allergic to nickel, in whom magnesium chloride but not sodium chloride, suppressed nickel sulphate-induced contact dermatitis.