The conditions for the heterologous expression of both untagged and His-tagged human sulfite oxidase in Escherichia coli have been optimized. Maximum production of active enzyme requires expression in a mob- cell strain at low levels of the inducer. Using these conditions, 3.9-5.6 mg of untagged and 15 mg of His-tagged sulfite oxidase were isolated per liter of cell culture. These represent significantly higher levels than previously reported for any molybdopterin-containing protein. High levels of enzyme activity and molybdenum incorporation were maintained despite the increase in yield, and no significant differences in kinetic properties were observed between the tagged and untagged sulfite oxidase. Additionally, the molybdenum domain of sulfite oxidase was expressed in a stable, active form as a His-tagged protein. The molybdenum domain was also expressed in the presence of tungstate to enable examination of the molybdopterin-tungsten form of sulfite oxidase.