Background/Aims: Defects in klotho gene expression in the mouse result in a syndrome that resembles human aging. We recently identified expression of klotho in a mouse inner medullary collecting duct (mIMCD3) cell line for the first time, and in the present study we explored the physiological relevance of the regulation of klotho expression in the presence of oxidant stress injury. Methods: Klotho expression was analyzed by real-time PCR, Western blot, and immuocytochemical staining during exposure to hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>). Overexpression of the klotho gene was induced by klotho adenoviruses, and the number of apoptotic cells was counted by flowcytometry. Results: Oxidant stress injury by H<sub>2</sub>O<sub>2 </sub>dose-dependently reduced klotho expression and diminished klotho staining. There were fewer apoptotic cells among the klotho-transfected cells than among the control cells. Conclusion: Klotho is expressed in cell line mIMCD3, and the klotho gene may be involved in the process of oxidative stress injury and apoptosis in this cell line.