The alpha-2-L-fucosyltransferase in human plasma has been freed from alpha-3-L-fucosyltransferase activity and purified approximately 200,000-fold by a series of steps involving ammonium sulphate precipitation, hydrophobic chromatography on Phenyl Sepharose 4B and affinity chromatography first on GDP-adipate-Sepharose and then on GDP-hexanolamine-Sepharose. The purified alpha-2-L-fucosyltransferase had a M(r) on gel filtration HPLC of 158,000 and showed optimal activity in the pH range 6.5-7.0. The enzyme transferred fucose equally well to Type 1 (Gal beta 1-3GlcNAc) and Type 2 (Gal beta 1-4GlcNAc) substrates but Type 3 (Gal beta 1-3GalNAc) structures were less efficient acceptors. Competition experiments indicated that a single enzyme species in the purified preparation was responsible for reactivity with the Type 1 and Type 2 structures. Thus the differences in conformation between the Type 1 and Type 2 disaccharides do not appear to influence the capacities of their terminal non-reducing beta-D-galactosyl residues to function as acceptor substrates for the alpha-2-L-fucosyltransferase expressed by the blood group H gene in haemopoietic tissue.