The incorporation of radioisotopically labeled fucose into a prominent fucosylated component, i.e., fucose-labeled amino acid fucoside 4c (FL4c), of human embryonic lung cells is markedly decreased in cell lines derived from human tumors. In the current study, we have extended the above observations by examining more closely related normal and transformed human cells, e.g., Wi38 cells and SV40-transformed Wi38 cells. We have found that the level of radioisotopically labeled fucose incorporated into FL4c of the SV40-transformed human embryonic lung cells is dramatically reduced as compared to their normal counterpart cells. Additionally, we have chemically characterized FL4c. FL4c was purified from monkey tissues using a combination of gel filtration chromatography, ion-exchange chromatography, and preparative thin-layer chromatography. Analysis of this material was accomplished by amino acid analyzer and by gas-liquid chromatography; fucose, glucosamine, and aspartic acid in molar ratios of approximately 1.0:1.0:1.0 were observed. Furthermore, alpha-L-fucosidase treatment of [3H]fucose-labeled FL4c revealed that the fucose was alpha-linked and in a terminal position. Similar treatment of [3H]glucosamine-labeled FL4c yielded a component that cochromatographed with N-acetylglucosaminylasparagine. The combined results of the structural studies are consistent with the structure of FL4c being alpha-fucosyl-N-acetylglucosaminylasparagine.