The magnesium salt of R-form lipopolysaccharide (LPS) from Klebsiella pneumoniae strain LEN-111 (O3-:K1-) that was prepared after the removal of cationic materials by electrodialysis formed essentially the same ordered hexagonal lattice structure with a lattice constant of 14 to 15nm as the original non-electrodialyzed preparation of the R-form LPS. When the magnesium salt was suspended in 50mM glycine buffer or Tris buffer at pH 1.4 to 9.5 and kept at 4C for 24hr, its content of Mg was markedly decreased, and its hexagonal lattice structure was changed to a swollen hexagonal lattice structure with extended lattice constants at pH 1.4 and to a loose mesh-like structure at pH 3.0 or higher. In the original non-electrodialyzed preparation of the R-form LPS, the release of Mg and disintegration of the hexagonal lattice structure did not occur by suspending in buffers at pH 1.4 to 8.5 at 4C for 24hr, but occurred only at pH 9.0 or higher. The results suggest that organic cations that can be removed by electrodialysis play some part in tight binding to Mg²⁺ and in stabilizing the ordered hexagonal assembly of the R-form LPS.