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The aim of this investigation was to study the corrosion behaviour and products of uncoated neodymium–iron–boron magnets in the presence of dental amalgam. Microcosm plaques were grown on discs of neodymium–iron–boron magnets or amalgam in a constant depth film fermentor. The biofilms were supplied with artificial saliva and growth was determined by viable counting.The results showed that the neodymium–iron–boron magnets corroded with an average daily weight loss of 0.115 ± 0.032 per cent. However, when the magnets were in close proximity to the amalgam the amount of corrosion was reduced to a daily loss of 0.066 ± 0.023 per cent. The highest loss of constituent elements from the corrosion products of the magnets was observed for iron. The composition of the microcosm plaques altered markedly between the two materials with less streptococci and more Veillonella spp. present in the biofilms grown on magnets in the presence of amalgam. The corrosion of neodymium–iron–boron magnets is limited and in the presence of amalgam is reduced further. This suggests that amalgam present in the mouth will not cause an increased clinical risk in terms of biocompatibility with neodymium–iron–boron magnets.