R-form lipopolysaccharide (LPS) from Klebsiella pneumoniae strain LEN-111 (O3-:K1-), which was precipitated by the addition of 2 volumes of ethanol containing 10mM MgCl₂ for the purification process, ultrastructurally exhibited membrane pieces consisting of an ordered hexagonal lattice structure with a lattice constant of 14 to 15nm. When the R-form LPS was suspended in 50mM tris (hydroxymethyl) aminomethane buffer (at pH 8.5) containing 1mM or higher concentrations of CaCl₂ and kept at 4C for 10 hr, the ordered hexagonal lattice structure of the R-form LPS was disintegrated and changed to an irregular rough, mesh-like structure. By treatment with CaCl₂, the content of Mg in the LPS was markedly decreased, and conversely, the content of Ca was increased to a level depending upon the concentration of CaCl₂. Results indicate that the addition of CaCl₂ to suspensions of the Mg-bound R-form LPS result in a tighter binding of Ca²⁺ to the R-form LPS and the release of Mg²⁺ from the R-form LPS, and as a consequence, destroys the Mg²⁺-induced ordered hexagonal lattice structure of the R-form LPS.