Using a monoclonal antibody (mAb), designated AMP1, a novel cell adhesion molecule was identified on rat astrocytes and oligodendrocytes. When confluent monolayers of cultured rat astrocytes were labeled with AMP1, the antigen was discretely localized to the cell surface in regions of cell-cell contacts. The antibody did not label embryonic rat cortical neurons plated on monolayers of neonatal astrocytes, indicating that the antigen is neither present on cultured neurons nor does it reorganize on the glial surface under the neurons. On immunoblots of astrocytic or brain proteins, mAb AMP1 recognized a 106 kDa protein. In the present paper, data are presented demonstrating that the AMP1 antigen is a cell adhesion molecule and is distinct from all the known cell adhesion molecules present on astrocytes: N-CAM, N-cadherin, or members of the beta 1 integrin family. We have tentatively termed this molecule 'glial cell adhesion molecule' (G-CAM).