Normal human plasma was found to contain beta 1-4N-acetylgalactosaminyltransferase catalyzing the transfer of N-acetylgalactosamine from UDP-GalNAc to 3'-sialyl-lactose, NeuAc alpha 2-3Gal beta 1-4Glc. The transferred N-acetylgalactosaminyl residue was cleaved from the desialylated reaction product by the beta-N-acetylhexosaminidase from jack beans. Methylation and hydrolysis of the desialylated reaction product yielded only 2,3,6-tri-O-methylgalactose and 2,3,6-tri-O-methylglucose as neutral sugars, indicating that the N-acetylgalactosaminyl residue was introduced at position C-4 of the galactosyl residue of 3'-sialyllactose. The enzyme required Mn2+ ions for its activity and showed a pH optimum between 6.5 and 8.5. By using a wide variety of oligosaccharides and glycoconjugates, the acceptor specificity of the beta 1-4N-acetylgalactosaminyltransferase was investigated. No detectable amount of N-acetylgalactosamine was transferred to either 6'-sialyllactose or lactose. The enzyme did not act on ganglioside GM3, NeuAc alpha 2-3Gal beta 1-4Glc-ceramide, suggesting that the hydrophobic ceramide portion of GM3 interferes with the enzyme reaction. On the other hand, glycoproteins carrying terminal NeuAc alpha 2-3Gal beta 1-4GlcNAc structures on their N-linked oligosaccharide chains, e.g. Tamm-Horsfall glycoprotein, were efficient acceptors.