Bis (methylthio) and methylthio amino derivatives of 1-methylpyridinium (and quinolinium)-2-dithioacetic acids, found to have protective activity in mice vs. lethal doses of γ-radiation, were converted to Cu(II) complexes. The complexes were isolated as double salts with CuCl₂, and some were sufficiently soluble to be assayed for superoxide dismutase (SOD) activity. This was done by measuring the inhibition of the superoxide reduction of nitroblue tetrazolium. A positive correlation was found in both the pyridinium and quinolinium series between the extent of SOD activity and the degree of radiation protection. This provides evidence for a mechanism of whole body radiation protection, based on the ability to dismutate superoxide. Copper complexes of a variety of other heterocyclic ligands showed much less ability to mimic SOD activity.