A polyclonal antibody against a borate-RG-II complex is raised in rabbits. The antibody recognized RG-II exclusively in cell wall polysaccharides. Immunochemical studies demonstrated that the epitope is ubiquitous in cell walls of all the cells in radish and rice roots, cultured tobacco cells, red clover root nodules, and lily growing pollen tubes. The label was denser in proximal to plasma membrane, and not detected in middle lamella, suggesting that borate may cross-link newly secreted pectic polysaccharides at the membrane-cell wall interface.