A recombinant human Lewis α(1,3/1,4)fucosyltransferase has been studied for its acceptor substrate specificity and used in the synthesis of sialyl LeX and derivatives. The tetrasaccharide sialyl Lewis x (LeX) (Neuα2,3Galβ1,4(Fucα1,3)GlcNAc) has been identified as a ligand for endothelial leukocyte adhesion molecule-1 (ELAM-1), a glycoprotein synthesized in response to inflammatory agents during tissue injury to promote adhesion of neutrophils, monocytes and a subpopulation of lymphocytes. Sialyl LeX has also been found on the surface of some tumor and cancer cells. Sialyl LeX and analogs or mimetics may therefore offer new opportunities for tumor diagnosis and for the treatment of inflammation. The biosynthetic glycosidic bond formations of sialyl LeX have been determined, with three glycosyltransferases acting in sequence to form the molecule. Two groups have recently reported the total chemical synthesis of sialyl LeX. We describe in this letter our preliminary studies on the acceptor specificity of a recombinant Lewis α(1,3/1,4)fucosyltransferase (FucT) and its application to the synthesis of sialyl LeX and derivatives. The enzyme FucT catalyzes the transfer of fucose (Fuc) from GDP-Fuc to position 3 or 4 of the GlcNAc moiety of an acceptor oligosaccharide such as β-galactosyl-N-acetylglucosamine. The substrate specificity of this enzyme, however, is not well understood.