The gene galE encoding UDP-galactose 4-epimerase was cloned into E. coli BL21(DE3) from the chromosomal DNA of E. coli strain K-12. High expression of the soluble recombinant epimerase was achieved in the cell lysate. In order to evaluate the use of this epimerase in enzymatic synthesis of important alpha-Gal epitopes (oligosaccharides with a terminal Gal alpha 1,3Gal sequence), a new radioactivity assay (alpha 1,3-galactosyltransferase coupled assay) was established to characterize its activity in producing UDP-galactose from UDP-glucose. Approximately 2700 units (100 mg) enzyme with a specific activity of 27 U mg(-1) protein could be obtained from one liter of bacterial culture. The epimerase was active in a wide pH range with an optimum at pH 7.0. This expression system established a viable route to the enzymatic production of alpha-Gal oligosaccharides to support xenotransplantation research.