Serological cross-reactivity among intact lipopolysaccharides (LPS) from 01 Vibrio cholerae Inaba 0-form (Inaba), Yersinia enterocolitica 0 9 (09), non-01 K cholerae serogroup Hakata (Hakata) and Vibrio bio-serogroup 1875 Variant (1875 Variant) (all of which share Inaba antigen factor C), as well as a total of six kinds of chemically modified LPS (three from 0 9 and three from Inaba) was demonstrated by passive hemolysis and passive hemolysis inhibition by using these LPS as antigen for sensitizing sheep red blood cells and as inhibitor. These intact as well as chemically modified LPS contained, in their 0 polysaccharide chain, a(l+2)-linked linear perosamine (4-amino-4,6-dideoxy-~-manno-pyranose) homopolymers with different N-acyl groups : their acyl groups comprise 3-deoxy-~-glycero-tetronyl (Inaba LPS), formyl ( 0 9 LPS), 3-hydroxypropionyl (1875 Variant LPS), acetyl (Hakata LPS and artificially introduced into Inaba and 0 9 LPS), propionyl and butyryl (both artificially introduced into Inaba and 0 9 LPS) groups. N-Deacylation of the a( 1+2)-linked N-(3-deoxy-~-gZycero-tetronyl)perosamine homopolymer of Inaba and the N-formyl one of 0 9 LPS resulted in virtual elimination of their serological reactivity with both homologous and heterologous antisera. Furthermore, when the resultant NH, groups of the N-deacylated perosamine homopolymers of both LPS were N-acylated with acetyl, propionyl or butyryl groups, they markedly recovered both of their serological reactivities. These results are compatible with the interpretation that the Inaba antigen factor C possessed by the four bacteria is substantially related to the common presence of N-acyl groups, regardless of their identity, residing in the perosamine residues constituting the 0 polysaccharide chain of their LPS. It was also indicated that the group antigen factor A of 01 K cholerae is substantially related to the 3-deoxy-~-glycero-tetronyl groups residing in the perosamine homopolymer of Inaba LPS.Keywords. 01 Vibrio cholerae ; lipopolysaccharide.Current taxonomy of Vibrio cholerae divides it into two groups, 01 and non-01 V cholerae on the basis of their heatstable somatic antigen (or 0 antigen); the former (cholera vibrio) is further subdivided into two major 0 forms, Ogawa and Inaba, and the latter (non-agglutinable vibrio) into from 0 2 to 0155 0 forms [l, 21. It is well established that the 0 antigen of 01 V cholerae consists of three antigenic factors, i.e. group antigen factor A, Ogawa antigen factor B and Inaba antigen factor C. The 0 antigenic structure of 01 V cholerae Ogawa 0 form (referred to hereafter as Ogawa) and Inaba 0 form (referred to as Inaba) are represented by antigenic formulae AB(c) and AC, respectively [3].Serological cross-reactivity of 01 K cholerae (a causative agent of cholera) was first recognized in agglutination and agglutinin-absorption tests with two other bacterial pathogens, Brucella abortus and Yersinia enterocolitica 0 9 [4-71. Recently, a further four non-cholera vibrios sharing an antigenic factor with 01 V cholerae we...