The preparation of fluorescently labelled and internally quenched UDP-Gal probes bearing a fluorescence emitter and a quencher is described. The rate of transfer using several galactosyltransferases was examined. Our results demonstrate that galactose-modified, sugar-nucleotide-modified and double modified UDP-Gal analogues are recognized as weak substrates by blood group B alpha-(1-->3) galactosyltransferase, alpha-(1-->3) galactosyltransferase and milk bovine beta-(1-->4) galactosyltransferase.