Incorporation of L-[3H]fucose into glycoproteins was studied in R2, the giant neuron in the abdominal ganglion of Aplysia. [3H]fucose injected directly into the cell body of R2 was readily incorporated into glycoproteins which, as shown by autoradiography, were confined almost entirely to the injected neuron. Within 4 h after injection, 67% of the radioactivity in R2 had been incorporated into glycoproteins; at least 95% of these could be sedimented by centrifugation at 105,000 g, suggesting that they are associated with membranes. Extraction of the particulate fraction with sodium dodecyl sulfate (SDS), followed by gel filtration on Sephadex G-200 and polyacrylamide gel electrophoresis in SDS revealed the presence of only five major radioactive glycoprotein components which ranged in apparent molecular weight from 100,000 to 200,000 daltons. Similar results were obtained after intrasomatic injection of [3H]N-acetylgalactosamine. Mild acid hydrolysis of particulate fractions released all of the radioactivity in the form of fucose. When ganglia were incubated in the presence of [3H]fucose, radioactivity was preferentially incorporated into glial cells and connective tissue. In contrast to the relatively simple electrophoretic patterns obtained from cells injected with [3H]fucose, gel profiles of particulate fractions labeled with [14C]valine were much more complex.