serological relationships between escherichia coli and salmonella smooth and roughmutant lipopolysaccharides as revealed by enzymelinked immunosorbent assay for human immunoglobulin g antiendotoxin antibodies reactions of sera from healthy blood donors to smooth and rough mutant lipopolysaccharides lps from escherichia coli o111b4 and salmonella minnesota which had been complexed with polymyxin b were determined by enzymelinked immunosorbent assay relative enzymelinked immunosorbent assay reactivities were examined for prima facie evidence of crossreactivity by analyses of correlation and relative scatter patterns of reactivity were interpreted in relation to published structures evidence was obtained for two dominant epitopes associated with the lipid a2keto3deoxyoctulosonic acid and heptose regions of the lps core sera demonstrated apparent exclusive antibody homogeneity in that given sera showed only one of the two possible specificities which occurred with equal frequency crossreactivity was found in the lipid a2keto3deoxyoctulosonic acid region for all lps crossreactivity was found in the heptose region for larger rough mutant s minnesota lps and smooth s minnesota lps and for e coli o111b4 lps but not for e coli rough mutant j5 lps whose heptose region appears to be different from those of the other organisms