The structures of FL4b and of two other related amino acid fucosides have been determined by a combination of methylation analysis and enzymatic digestion. Additionally, the anomeric configurations of the carbohydrate moieties of FL4a, previously shown to be glucosyl(1 leads to 3)fucosyl 1 leads to threonine (Steiner, S., Via, D. P., Klinger, M., Larriba, G., Sramek, S., and Laine, R. (1978) in Glycoproteins and Glycolipids in Disease Processes (Walborg, E. F., Jr., ed) pp. 378-403, American Chemical Society, Washington, D. C.) have been determined by enzymatic digestion. The results indicate that the structures are: FL3a, fucosyl alpha 1 leads to threonine; FL3b, fucosyl alpha 1 leads to serine; FL4a, glucosyl beta 1 leads to 3 fucosyl alpha 1 leads to threonine; and FL4b, glucosyl beta 1 leads to 3 fucosyl alpha 1 leads to serine. FL4a, which appears to have the same structure as a component from human urine (Hallgren, P., Lundblad, A., and Svensson, S. (1975) J. Bil. Chem. 250, 5312-5314), and FL4b are highly unusual in that they contain fucose in a nonterminal position. The fucosyl-serine linkage found in compounds FL3b and FL4b is a novel structure.