We have previously used a gene-transfer scheme to isolate a human genomic DNA fragment that determines expression of a GDP-L-fucose:I3-D-galactoside 2-a-L-fucosyltransferase [a(1,2)FT; EC 2.4.1.69]. Although this fragment determined expression of an a(1,2)FT whose kinetic properties mirror those ofthe human H blood group a(1,2)FT, their precise nature remained undefined. We describe here the molecular cloning, sequence, and expression ofa human cDNA corresponding to these human genomic sequences. When expressed in COS-1 cells, this cDNA directs expression of cell surface H structures and a cognate a(1,2)FT activity with properties analogous to the human H blood group a(1,2)FI. The cDNA sequence predicts a 365-amino acid polypeptide characteristic of a type II transmembrane glycoprotein with a domain structure analogous to that of other glycosyltransferases but without significant primary sequence similarity to these or other known proteins. To directly demonstrate that the cDNA encodes an a(1,2)FT, the COOH-terminal domain predicted to be Golgi-resident was expressed in COS-1 cells as a catalytically active, secreted, and soluble protein A fusion