Nectadrin, the cell surface glycoprotein recognized by the novel mAb 79, was found to be immunologically identical to the heat-stable antigen (HSA). It is a glycoprotein with a polypeptide core of only 30 amino acids and a very high carbohydrate content (Wenger, R. H., M. Ayane, R. Bose, G. Köhler, and P. J. Nielsen. 1991. Eur. J. Immunol. 21:1039-1046). Immunocytological studies using cultured splenic B-lymphocytes, neuroblastoma cells, and cerebellar cells indicated that nectadrin is preferentially expressed at sites of cell-cell contact. Purified nectadrin and monoclonal nectadrin antibody 79, but not other monoclonal nectadrin antibodies, inhibited the aggregation of B-lymphocytes by 70%, suggesting that nectadrin may act as a cell adhesion molecule. Nectadrin was purified from a mouse lymphoma cell line in two forms of 40-60 and 23-30 kD. The lower molecular weight form appears to be generated from the higher molecular weight form by degradative removal of saccharide residues characteristic of complex type oligosaccharide side chains. Latex beads coated with purified nectadrin aggregated and the rate of their aggregation depended on the molecular form of nectadrin, with the larger form being more potent than the smaller one in mediating bead aggregation. Nectadrin thus appears to be a self-binding cell adhesion molecule of a structurally novel type in that its extensive glycan structures may be implicated in mediating cell adhesion.