Enterobacterial common antigen (ECA) is a conserved surface antigen characteristic for Enterobacteriaceae. It is consisting of trisaccharide repeating unit, →3)-α- d-Fuc p4NAc-(1→4)-β- d-Man pNAcA-(1→4)-α- d-Glc pNAc-(1→, where prevailing forms include ECA linked to phosphatidylglycerol (ECA PG) and cyclic ECA (ECA CYC). Lipopolysaccharide (LPS)-associated form (ECA LPS) has been proved to date only for rough Shigella sonnei phase II. Depending on the structure organization, ECA constitutes surface antigen (ECA PG and ECA LPS) or maintains the outer membrane permeability barrier (ECA CYC). The existence of LPS was hypothesized in the 1960–80s on the basis of serological observations. Only a few Escherichia coli strains (i.e., R1, R2, R3, R4, and K-12) have led to the generation of anti-ECA antibodies upon immunization, excluding ECA PG as an immunogen and conjecturing ECA LPS as the only immunogenic form. Here, we presented a structural survey of ECA LPS in E. coli R1, R2, R3, and R4 to correlate previous serological observations with the presence of ECA LPS. The low yields of ECA LPS were identified in the R1, R2, and R4 strains, where ECA occupied outer core residues of LPS that used to be substituted by O-specific polysaccharide in the case of smooth LPS. Previously published observations and hypotheses regarding the immunogenicity and biosynthesis of ECA LPS were discussed and correlated with presented herein structural data.