5'-O-beta-D-Galactosyl-floxuridine, a potential novel prodrug, was synthesized with a yield of 75% through beta-galactosidase-catalyzed transgalactosylation. This enzyme displayed absolute regioselectivity toward the 5'-position of floxuridine. For the reaction, the optimal conditions were pH 6.5 at 45 degrees C for 60 h with floxuridine to o-nitrophenyl-beta-D-galactoside at 2:1 (mol/mol). Under these conditions, the initial reaction rate and the maximum yield were 0.28 mM h(-1) and 75%, respectively.