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This study was conducted to evaluate the biocompatibility of titanium-nickel shape memory alloy used as a medical implant material. The authors carried out the following electrochemical corrosion test and in vivo and in vitro biological tests for the alloy and some metal and alloys clinically used previously to compare the intensities concerned with the biological reactions, that is, (1) anodic polarization test for the alloy in a quasi-body fluid, (2) cell proliferation tests for pure titanium (cp Ti), pure nickel (cp Ni), SUS316L stainless steel, titanium-6 mass% aluminium-4 mass% vanadium (Ti-6Al-4V), and titanium-55 mass% nickel (Ti-55Ni) by using of L929 fibroblastic cells, (3) Lactate dehydrogenaze (LDH), human interlukin-1 (hIL-1), and human tumor necrosis factor-(hTNF-) biochemical assays by using of U937 human macrophages administered the corrosion products of these alloys for the cells, (4) measurement of the mount of excretions of the metallic corrosion products of Ti-55Ni, SUS316L stainless steel, and Ti-6Al-4V with urine and feces injected into the abdomen cavity of Wistar rats, and (5) tissue reaction observations for SUS316L, Ti-55Ni, and cp Ni wires implanted along the femoral bone axis of the rats.The following results were obtained. (1) The pitting corrosion potentials of Ti-55Ni alloy was drastically improved by the aging treatment. (2) In the case of Ti-55Ni alloy, the inflammatory cytokines, hIL-6 and hTNF-were suppressed to lower levels compared with Ti-6Al-4V alloy. (3) Corrosion products prepared from the titanium alloys were stable in the body. Then it is very hard to eliminate the titanium ions with urine and feces. (4) Ti-55Ni alloy was shown an excellent biocompatibility evaluated by the in vivo implantation test, because of the stable passive film formed on the surface and protected the metal ion release to the surrounding tissue.