We have demonstrated previously that there is a marked decrease in the level of labeled fucose incorporated into an amino acid fucoside, i.e., fucosyl-N-acetylglucosaminylasparagine (FL4c) in SV40-transformed human embryonic lung cells (SV40-WI-38) as compared to WI-38 cells (Morton, P.A., Klinger, M. M., and Steiner, S. Cancer Res., 42: 3022-3027, 1982). In the current study, we have observed that the reduction in labeled fucose incorporated into FL4c in the SV40-WI-38 cells is paralleled by reduced chemical quantity of that component. [3H]-Fucose pulse/chase and long-term fucose labeling/chase studies, in some instances in the presence of tunicamycin, have revealed that FL4c is a relatively stable end produce of N-linked-type glycoprotein metabolism. The relative metabolic stability argues against breakdown of FL4c as the basis for the markedly reduced level in the SV40-WI-38 cells. However, the transformed cells manifested an almost 3-fold higher level of alpha-L-fucosidase activity when p-nitrophenyl-alpha-fucoside was used as substrate. These results raise the possibility that the parent glycoprotein of FL4c might be more rapidly catabolized in the SV40-WI-38 cells than in the WI-38 cells. Whatever the biochemical basis for the decrease in level of FL4c in transformed human cells, it would seem to underlie a difference between normal and transformed cells in membrane glycoprotein catabolism.