The present study reports a control on the biosynthesis of fucosylated structures, serving as ligands for selectins by demonstrating the potential of 3-sialyl or 3-sulfo Gal beta 1, 3/4GlcNAc beta-containing glycoconjugates as high affinity substrates for alpha 1, 3/4-L-fucosyltransferases and as substrate inhibitors at higher concentrations. The synthetic sulfated saccharides and the triantennary sialoglycopeptide from fetuin were potent competitive inhibitors of the transfer of fucose to non-anionic saccharide acceptors and the corresponding triantennary asialoglycopeptide respectively catalyzed by a partially purified alpha 1, 3/4-L-fucosyltransferase preparation from Colo 205 (specific activity:transfer of 113.1 nmol Fuc to 2'-FucosylLacNAc per h per mg protein); Ki for the inhibitions by triantennary sialoglycopeptide, 3-SulfoGal beta 1, 3GlcNAc beta-0-Allyl and a copolymer from 3-SulfoGal beta 1, 3GlcNAc beta-0-Allyl and acrylamide were 51.9 microM, 500 microM and 67.0 microM, respectively. Further, the alpha 1,3-specific anionic acceptor, 3'-SulfoLacNAc, also inhibited the alpha 1,4- activity; Km for the alpha 1,4-specific acceptor, 2-methylGal beta 1, 3GlcNAc beta-0-Bn increased from 0.40 mM to 1.35 mM in presence of 3.0 mM 3'-sulfoLacNAc, whereas Ki for the mutual inhibition of alpha 1,3-activity by the former was found to be high (3.64 mM). Furthermore, the phenomenon of substrate inhibition, serving as acceptors at lower concentrations and as inhibitors at higher concentrations, was exhibited by the anionic acceptors; the Hill plots gave the Ki values 342.7 microM, 13.03 mM and 13.36 mM respectively for fetuin triantennary sialo glycopeptide, 3'-sulfoLacNAc and 3-sulfoGal beta 1, 3GlcNAc beta-0-Allyl.