The surface glycopeptides of human trophoblastic cells have been compared with those of fetal cells from the same embryos using double-labeling methods with isotopes of L-fucose and D-glucosamine. A faster eluting, neuraminidase-sensitive, fraction was observed on Sephadex chromatography of the trophoblast spectra when D-glucosamine was used as precursor. Labeling with fucose did not appear to result in any differences, thus suggesting that the glycopeptides characertistic of trophoblast contained glucosamine-derived metabolic products, including sialic acid, but excluding fucose. This increased sialylation is similar to, but not identical with, modifications observed in neoplastic cells, and on this basis it is postulated that two species of glycopeptides may be involved in atypical cellular behavior. The first contains sialic acid and other sugars excluding fucose, and is associated with localized cellular growth and invasion. The second contains both sialic acid and fucose and is characteristic of neoplastic cells.