L-Gulono-gamma-lactone oxidase (GLO), an enzyme that is missing in scurvy-prone animals, was produced in COS-1 cells by transfection with a minigene constructed from pSVL vector and rat GLO cDNA. A functional GLO protein was expressed, which was indistinguishable from rat GLO in molecular size. The microsomes obtained from the transfected COS-1 cells showed GLO activity comparable to that in rat liver microsomes.