An R-form lipopolysaccharide (LPS) from Klebsiella pneumoniae strain LEN-111 (03-:K1-) formed crystals, whose shapes were elongated hexagonal plates, trapezoid plates, and rhomboid plates, and whose greatest dimensions were 3.1 x 0.8 microns, when it was suspended in 50 mM Tris buffer at pH 8.5 containing 5 mM MgCl2 and kept at 4 C for as long as 870 days. K. pneumoniae LEN-111 synthesized LPS molecules possessing incomplete repeating units of the O-antigenic polysaccharide portion besides the R-form LPS because of a leaky characteristic, but crystals consisted exclusively of the R-form LPS. Although the size of crystals was not large enough for X-ray analysis and limited crystallographic information was available, it was suggested that the crystals consist of hexagonal lattices with an alpha axis of 4.62 A and c axis of 79.8 +/- 2.6 A. The present results showed that R-form LPS lacking the O-antigenic polysaccharide portion tends to form crystals during long-term incubation in Tris buffer at pH 8.5 containing MgCl2 at 4 C.