Discs of five high-purity metals, cobalt, nickel, copper, aluminium and lead have been implanted intramuscularly in rats and the response observed histologically for period up to 52 weeks. A reproducible but different response was observed with each metal. Whenever corrosion occurred, as with copper, nickel and some cobalt specimens, the implants became loose. In the absence of corrosion, the implants were firmly held within a more confined capsule. A minimal response was seen with lead, implying normally toxic metals do elicit an immune response whilst some, especially copper and nickel appear to render the host more susceptible to disease. The implants appear to have a profound effect on the immediate vasculature, are able to cause a prolonged polymorphonuclear response in the same way as bacteria, are associated with varying amounts of haemosiderin laden macrophages but not with giant cells. The animals appear to be able to deal with bacteria introduced at surgery without hindrance from the metal.