The concanavalin A--binding glycoproteins of the rat synaptic junction were isolated by affinity chromatography. These glycoproteins had molecular weights of 160,000, 123,000, 110,000, and 95,000. The tryptic peptide maps of these glycoproteins showed that the three largest glycoproteins contained a high percentage of identical peptides. This indicates that the amino acid sequences of these glycoproteins have a high degree of homology. The 95,000-dalton glycoprotein was unrelated to the other three. These findings suggest that homologous glycoproteins may participate in synapse formation or maintenance, or both.