The purpose of this study was to characterize the surface oxide films on a Co-Cr-Mo alloy located in various environments to estimate the reconstruction of the film in the human body. Five kinds of specimens were prepared according to the following methods: polishing in deionized water. autoclaving, immersion in Hanks' solution, immersion in a cell culture medium, and incubation with cultured cells. X-ray photoelectron spectroscopy (XPS) was performed to estimate the compositions of the surface oxide film and substrate and the thickness of the film. The surface oxide was about 2.5 nm. thick and contained a large amount of OH-. Calcium phosphate was formed after immersion in the Hanks' solution and the culture medium and incubation with the cultured cells. Cobalt was dissolved during immersion in Hanks' solution and the cell culture medium and incubation in a cell culture. After the dissolution, the surface oxide consisted of chromium oxide (Cr3+) containing molybdenum oxide (Mo4+, Mo5+, and Mo6+). Results from angle-resolved XPS revealed that chromium and molybdenum were more widely distributed in the inner laver than in the outer layer of the oxide film, The results in this study suggest that Co-Cr-Mo alloy releases cobalt and that the surface oxide film consisting of chromium and molybdenum is reconstructed in living tissues.