Chemical and serological studies were performed with the lipopolysaccharide (LPS) from Vibrio cholerae O144 (O144). The LPS of O144 contained D-glucose, D-galactose, L-glycero-D-manno-heptose, D-fructose, D-quinovosamine (2-amino-2, 6-dideoxy-D-gluco-pyranose) and L-perosamine (4-amino-4, 6-dideoxy-L-manno-pyranose). The perosamine, a major component sugar of the LPS from O144, was in an L-configuration, as is also the case in the LPS from V. cholerae O76 (O76), in contrast to the D-configuration of the perosamine in the LPS of V. cholerae O1. A structural analysis revealed that the O polysaccharide chain of the LPS from O144 is an α(1→2)-linked homopolymer of (R)-(-)-2-hydroxypropionyl-L-perosamine. The serological cross-reactivity between O144 and O76 was clearly revealed by cross-agglutination and cross-agglutinin absorption tests with whole cells, as well as by passive hemolysis tests with sheep red-blood cells that had been sensitized with the LPS from O144 and O76. In contrast, in passive hemolysis tests, the LPS of O144 did not cross-react serologically with the LPSs from other strains such as V. cholerae O1 (Ogawa and Inaba), V. cholerae O140, Vibrio bio-serogroup 1875 (Original and Variant) and Yersinia enterocolitica O9. The LPSs from these strains consist of O polysaccharide chains composed of α(1→2)-linked homopolymers of D-perosamine with various N-acyl groups, and they share the Inaba antigen factor C of V. cholerae O1 in common. The results obtained in this study demonstrate that the absolute configuration of the perosamine residue in homopolymers plays a very important role in the expression of the serological specificity of the Inaba antigen factor C of V. cholerae O1.