Sera from ten patients with positive brucella serology were used to investigate antibody cross-reactions between the O-antigens of Escherichia coli 0157 and Yersinia enterocolitica 09. SDS-PAGE profiles of lipopolysaccharide (LPS), purified from strains of E. coli 0157 and Y. enterocolitica 09, were reacted with sera by immunoblotting. All ten sera contained antibodies which bound to the LPS of E. coli 0157, and five of these sera also contained antibodies which bound to the LPS of Y. enterocolitica 09. Absorption studies using these five cross–reacting sera indicated the existence of at least three epitopes exposed on the O–antigens of E. coli 0157 and Y. enterocolitica 09. One antigen binding site appeared to be exposed on the LPS of both organisms, while one epitope was exposed on the LPS of E. coli 0157 only, and another on the LPS of Y. enterocolitica 09 only.