The antigenic epitope detected on myelin associated glycoprotein (MAG) by the monoclonal antibody HNK-1 (Leu 7) was sensitive to degradation by trifluoromethane-sulfonic acid (TFMS) and is therefore probably carbohydrate in nature. This antigen was found to be widely distributed within the rat and chicken embryonic nervous system and was present on cultured central and peripheral neurons (100%), oligodendrocytes (100%) and astrocytes (70-80%) as detected by double marker immunofluorescence. The antigen could be removed from cultured neurons by trypsinization and its resynthesis was blocked by cycloheximide, suggesting that the carbohydrate epitope detected by HNK-1 was attached to a de novo synthesized protein. Several molecular species were detected on Western blots of detergent extracts from 13-15d rat embryonic brain and neuron-enriched cultures from chick spinal cord and dorsal root ganglia. Protein components with molecular weights in the ranges of 90-100 kd to 280 kd were observed and comprise a family of glycoproteins containing the HNK-1 reactive carbohydrate epitope present on MAG. These glycoproteins could play a role in intercellular interactions within the developing nervous system.