87 subjects sensitive to both nickel sulfate and palladium-chloride pet., were contemporaneously patch retested to nickel sulfate 5% pet., metallic palladium chloride 1% pet. and to palladium chloride 1% aq. Whilst all subjects reacted to nickel sulfate and palladium chloride pet., only 3 reacted to palladium chloride aq. No positive reactions were found to metallic palladium. The negative results to palladium chloride aq. are probably due to the formation of a new palladium ion (PdCl4)2-, achieved on adding an amount of hydrocloric acid to the aqueous solution of PdCl2. The findings seem to demonstrate that the allergic reaction to palladium depends on the arrangement of the metal electrons. The sensitization to palladium does not seem to be dependent on the element itself but on the complexes formed by the different compounds. The concomitant reactions to nickel and palladium ions could be dependent on the generation of similar complexes between the ions and the skin proteins.