Glycosyltransferase activities (ectogalactosyl, ectofucosyl and ectosialyl) were studied at the external surface of exclusively neuronal cultures. An appropriate methodology gave the possibility to eliminate sources of errors due to the hydrolysis of nucleotide sugar substrates or due to cellular uptake of free sugars. Ovomucoid and asialofetuin coupled to Sepharose and Ultrogel beads were used as exogenous substrate to circumvent possible substrates pinocytosis. Ectoglycosyltransferase activities were studied as function of protein concentration, incubation time and amount of bead coupled exogenous acceptors. The data show that these enzymes are present at the external surface of the neuronal membrane; their possible role in cell - cell interactions is suggested.