C-CAM is a Ca(2+)-independent cell adhesion molecule (CAM) belonging to the immunoglobulin superfamily. Addition of chemical cross-linkers to isolated rat liver plasma membranes, intact epithelial cells and purified preparations of C-CAM stabilized one major C-CAM-containing product whose apparent molecular mass was approximately twice that of the C-CAM monomer. The failure to detect additional proteins after cleavage of the cross-linked species demonstrated that C-CAM exists as non-covalently linked dimers both in solution and on the cell surface. Dimerization occurred to the same extent in adherent monolayers and in single cell populations, indicating that dimer formation was the result of cis-interactions within the membranes of individual cells. Using isoform-specific anti-peptide antibodies, both C-CAM1 and C-CAM2 were found to be involved in dimerization, forming predominantly homo-dimeric species. Both calmodulin and Ca2+ ionophore modulated the level of dimer formation, suggesting a role for regulated self-association in the functional activity of C-CAM.