Metal release from implantable metals and the properties of oxide films formed on alloy surfaces were analyzed, focusing on the highly biocompatible Ti-15Zr-4Nb-4Ta alloy. The thickness and electrical resistance (R p ) of the oxide film on such an alloy were compared with those of other implantable metals. The quantity of metal released during a 1-week immersion test was considerably smaller for the Ti-15Zr-4Nb-4Ta than the Ti-6Al-4V alloy. The potential (E 10 ) indicating a current density of 10 µA cm −2 estimated from the anodic polarization curve was significantly higher for the Ti-15Zr-4Nb-4Ta than the Ti-6Al-4V alloy and other metals. Moreover, the oxide film (4-7 nm thickness) formed on the Ti-15Zr-4Nb-4Ta surface is electrochemically robust. The oxide film mainly consisted of TiO 2 with small amounts of ZrO 2 , Nb 2 O 5 and Ta 2 O 5 that made the film electrochemically stable. The R p of Ti-15Zr-4Nb-4Ta was higher than that of Ti-6Al-4V, i.e. 0.9 cm 2 in 0.9% NaCl and 1.3 cm 2 in Eagle's medium. This R p was approximately five-fold higher than that of stainless steel, which has a history of more than 40 years of clinical use in the human body. Ti-15Zr-4Nb-4Ta is a potential implant material for long-term clinical use. Moreover, E 10 and R p were found to be useful parameters for assessing biological safety.Keywords: Ti-15Zr-4Nb-4Ta alloy, implantable metals, passive film, field-emission transmission electron microscopy, angle-resolved x-ray photoelectron spectroscopy, electrochemical stability evaluation