Experimentally demonstrated interactions between Ni2+ and Mg2+ were examined in human beings. 110 subjects patch-test-positive to 10 microliters aq. NiSO4 0.1 M were subdivided into groups of 30, 50 and 30 people. Each subgroup was tested to 10 microliters NiSO4 0.1 M solution as a control and to mixed solutions containing NiSO4 0.1 M together with, respectively MgSO4 0.1 and 0.3, 0.3 and 0.5 and 0.5 and 1 M. On increasing the applied concentrations of MgSO4, the % of patients with reduced or suppressed nickel reactions, with 1 exception, proportionally increased. The exception concerned testing with 0.5 M, where a paradoxically exacerbating increase in nickel reactions was seen in a majority of nickel-sensitive subjects. MgCl2 aq. at 0.3, 0.5 and 1 M concentrations was not able to reduce the cutaneous patch test positive reactions to NiCl2 0.1 M in 25 sensitive patients. On increasing the applied concentrations of MgCl2, both the number and intensity of patch test reactions to NiCl2 proportionally increased. A supposed rôle of the sulfate and chloride counterions in the penetration of nickel was examined in 30 NiSO4 5% patch-test-positive patients, testing to 10 microliters of aq. NiSO4 0.1 M, NiCl2 0.1 M, NiSO4 0.1 M + MgCl2 0.3 M, NiCl2 0.1 M + MgSO4 0.3 M, Na2SO4 0.3 M, NaCl 0.3 M, NiSO4 0.1 M + Na2SO4 0.3 M, NiCl2 0.1 M + NaCl 0.3 M. The findings suggest that the addition of sulfate or chloride to nickel could determine the formation of different Ni complexes directed toward different targets, one Mg(2+)-dependent, the other Mg(2+)-independent.