Structures of N-linked sugar chains are species and tissue specific and change in the course of tumorigenesis. Sialyl linkages of human placental glycoproteins are exclusively Neu5Ac alpha2-->3Gal, whereas Fuc alpha1-->2Gal and Neu5Ac alpha2-->6Gal residues are expressed in human chorionic gonadotropin and alkaline phosphatase, which are produced in human choriocarcinoma JEG-3 and BeWo cells. In the present study, to elucidate the enzymological and molecular biological basis of the structural changes that occur in the course of tumorigenesis, alpha1-->2 fucosyltransferase, alpha2-->3 and alpha2-->6 sialyltransferase activities, and the expression levels of the corresponding mRNAs were measured. The alpha2-->3 sialyltransferase activity did not change as a result of tumorigenesis; however, the alpha2-->6 siayltransferase activity and alpha1-->2 fucosyltransferase activity in JEG-3 and BeWo cells increased to levels several times higher than those in placenta Competitive PCR analysis showed that the expression levels of mRNA encoding alpha1-->2 fucosyltransferase and mRNA encoding alpha2-->6 sialyltransferase increased significantly as a result of tumorigenesis, indicating that such structural changes are regulated at the level of transcription of these glycosyltransferase genes.