A high pH anion exchange chromatographic (HPAEC) system for the separation of isomeric sialo-oligosaccharide products was developed. Employing this system, using Gal beta 1-->4GlcNAc beta 1-->2Man alpha 1-->6Man beta 1-->4GlcNAc as a substrate, a Gal beta 1-->4GlcNAc-R alpha 2-->3-sialyltransferase activity was detected for the first time in human liver. This activity is expressed together with the prevalent alpha 2-->6-sialyltransferase. Furthermore, in addition to the major alpha 2-->3-sialyltransferase, a low but distinct activity of alpha 2-->6-sialyltransferase was detected in human placenta. This activity could not be found by methods based on methylation analysis or high resolution NMR spectroscopy. It is concluded that HPAEC, in combination with the use of the pentasaccharide as an acceptor substrate, is suited for the specific detection of minor, Gal beta 1-->4GlcNAc-specific sialyltransferase activities.