.beta.-Galactosides are synthesized using a transglycosylation reaction catalyzed by .beta.-galactosidase. The reaction employs a carbohydrate donor having a glycosidic leaving group attached to its anomeric carbon and an oxo group attached to the C-6 carbon. Strong leaving groups are preferred over weak leaving groups. The method can be carried out in aqueous solution without organic solvents to give the transglycosylation product in high yields and high regioselectivity. The synthesis of lactosamine using this methodology with galactose oxidase (GO) and .beta.-galactosidase has been accomplished. (FIG. 3). The methodology affords simple reaction conditions and minimal purification steps. In addition, the intermediate substrates maintain high stability, the process affords high yields and the enzymes and reagents employed are commercially available with high stability and low costs.