Neurite outgrowth promoting properties of neural cell surface proteins can be assessed by immobilizing isolated membrane proteins on nitrocellulose-coated petri dishes. Using this method, we have identified a unique cell surface antigen, designated P84, as a new neural cell adhesion molecule. Immunoaffinity purified P84 contains three polypeptides with molecular weights of 167, 85, and 66 kDa. When spotted onto nitrocellulose-coated plates, P84 supports adhesion of mouse cerebellar neurons and neurite outgrowth. Glial cell attachment was also observed. Intact monoclonal antibodies directed against P84 inhibit adhesion and outgrowth on a P84 substrate. This antigen is found on the surfaces of neurons in cultures of cerebellar cells. It is also found on a subclass of unidentified flat cells. P84 is not found on oligodendrocytes or GFAP-positive astrocytes. As early as E9, P84 could be detected in the floor plate region of the spinal cord. This pattern persists throughout embryonic development. Postnatally, widespread expression of P84 is observed in a variety of CNS tissues.