The R-form lipopolysaccharide (LPS) from Escherichia coli K-12, from which cationic material had been removed by electrodialysis and the pH of which had fallen to 3.6, formed a rough hexagonal lattice structure with the lattice constant of about 19nm. The rough hexagonal structure was maintained in buffers at pH 5 or lower but disintegrated into the ribbon-like structures in buffers at pH 6 or higher. However, in the presence of 10mM Mg²⁺, the hexagonal lattice structure was not disintegrated even at alkaline pH levels but conversely it became more dense. At pH 8.3 to 8.9, the hexagonal lattice structure with the shortest lattice constant (15nm) was formed. The same optimal pH levels were obtained for formation of the dense hexagonal lattice structure (lattice constant, 14 to 15nm) by the electrodialyzed LPS from Klebsiella pneumoniae strain LEN-111 (O3-:K1-). The ability of Mg²⁺ to induce formation of the dense hexagonal lattice structure of the K-12 LPS depends upon the presence of buffers showing the optimal pH levels, since a very high concentration of Mg²⁺ such as 500mM was required for the lattice formation in distilled water. The amount of the magnesium bound to the K-12 LPS did not significantly differ throughout the pH range of 3 to 9. Therefore, the optimal pH range is another essential factor for formation of the dense hexagonal lattice structure of the LPS in addition to binding of the magnesium to the LPS.