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In vivo degradation of magnesium alloy implant, the bone response to the magnesium, and the effect of the degradation of magnesium on the blood composition and organs were investigated by using light microscopy and scanning electronic microscopy with energy dispersive spectrum. Magnesium alloy showed different degradation rates in the marrow channel and the cortical bone. More degradation of magnesium implant was observed in the marrow channel than in the cortical bone. New bone tissue formed around the magnesium implants after 6 weeks implantation but no fibrous capsule was found. There existed two distinct layers separating the new bone tissue from the magnesium implant. On the magnesium implant side, crystalline magnesium calcium phosphate formed oil the surface of the implant due to the reaction between the implant and blood or body fluid. On the new bone tissue side was a 10-30-mu m membrane comprising two distinct layers with many fibroblasts in the layer close to the new bone tissue. The new bone was in tight contact with the implant through the membrane and phosphate layer due to the good osteoconductivity of the phosphate layer. After 10 and 26 weeks postimplantation, more new bone tissues as well as the membrane were found around the implant. However, no apparent increase in the thickness of the membrane was observed with the increasing of the implantation duration. Blood examination has shown that the degradation of the magnesium implant caused little change to blood composition but no disorder to liver or kidneys. (C) 2008 Wiley Periodicals, Inc. J Biomed Mater Res 90A: 882-893, 2009