An analog (1) of GDP-fucose, where C-6 is derivatized with an eight-atom spacer terminating in a primary amino group, was chemically synthesized. This amino group in sugar nucleotide1can be acylated using anN-hydroxysuccinimide ester of biotin and it can be coupled to another molecule that also contains an amino group using squaric acid diethyl ester as the coupling reagent. In this way, biotin and a blood group A-active trisaccharide were linked to C-6 of fucose in GDP-fucose. Both complex sugar nucleotides thus prepared were active as donors for a human milk fucosyltransferase, which transferred the derivatized α-linked fucose residue to a glycoside ofN-acetyllactosamine, thus labeling this sequence with either biotin or the blood-group A trisaccharide. Compound1is proposed as a general and versatile reagent which should permit the addition of biological probes to the sugar chains of cell surface glycoproteins or glycolipids.