A strong and constitutive expression vector of Escherichia coli beta -glucuronidase with the isocitrate dehydrogenase promoter has been developed for producing a large amount of recombinant protein. More than 95% pure enzyme was obtained by a four step purification procedure-ammonium sulfate precipitation, DEAE ion-exchange chromatography, Superose 12 gel filtration, and hydroxyapatite steric ion-exchange chromatography. The overexpressed gene can produce 23 mg of pure enzyme from one liter of bacterial culture.