Lipopolysaccharide (LPS) was isolated from a strain of Escherichia coli O157 by two different methods and used to prepare antisera in rabbits. LPS prepared by the proteolytic digestion of whole cells was identical to LPS prepared with hot-phenol, as shown by SDS-PAGE and silver staining. Antisera from rabbits receiving these LPS preparations contained high titred antibodies (> 10(5)) of the IgM class to E. coli O157 LPS.