The B gene-specified alpha-D-(1----3)-galactosyltransferase, isolated from the serum of a blood-group B individual, was used to catalyse the transfer of N-acetyl-D-galactosamine from UDP-N-acetyl-D-galactosamine to the blood-group H-active trisaccharide 2'-fucosyllactose. The biosynthetic product had blood-group A activity and its structure was confirmed as alpha-D-GalpNAc-(1----3)-[alpha-L-Fucp-(1----2)]-beta-D-Galp-(1--- -4)-D-Glc by methylation analysis and high-resolution 1H-n.m.r. spectroscopy. This tetrasaccharide was structurally and serologically identical with that made from the same donor and acceptor substrates when the blood-group A gene-specified alpha-D-(1----3)-N-acetylgalactosaminyltransferase was used as the enzyme source. The enzyme encoded by the B gene at the blood group ABO locus thus has overlapping donor substrate specificity with the enzyme encoded by the allelic A gene, and this property confers upon the B gene-specified alpha-D-1----3)-galactosyltransferase the potential to synthesise blood-group A-active structures.