Diverse glycoproteins of cell surfaces and extracellular matrices operationally termed 'adhesion molecules' are important in the specification of cell interactions during development, maintenance and regeneration of the nervous system. These adhesion molecules have distinct functions involving different cells at different developmental stages, but may cooperate when expressed together. Families of adhesion molecules which share common carbohydrate domains do exist, despite the structural and functional diversity of these glycoproteins. These include the Ca2+-independent neural adhesion molecules: N-CAM, myelin associated glycoprotein (MAG) and L1. L1 is involved in neuron-neuron adhesion, neurite fasciculation, outgrowth of neurites, cerebellar granule cell migration, neurite outgrowth on Schwann cells and interactions among epithelial cells of intestinal crypts. We show here that in addition to sharing carbohydrate epitopes with N-CAM and MAG, L1 is also a member of the immunoglobulin superfamily. It contains six C2 domains and also shares three type III domains with the extracellular matrix adhesion molecule fibronectin.