S U M M A R YSalmonella typhimurium mutants, called Felix 0-resistant (FOR), selected for resistance to phage Felix 0 (FO) which has its receptor in the core lipopolysaccharide (LPS), retain most of the properties of the smooth parent strain (MacPhee, Krishnapillai, Roantree & Stocker, 1975). LPS extracted from one parent and two FOR strains by the phenol-water and the phenol-chloroform-light petroleum methods have been subjected to passive haemagglutination inhibition and methylation analysis. The amount of LPS, the amount of 0-specific sugars in the LPS, and the average length of the 0 chains were almost the same in parent and mutant strains. Neither passive haemagglutination nor methylation analysis revealed the presence of incomplete cores in the mutant strains. Determinations of the rates of attachment of P22 (receptor in 0 chain) and FO phages to whole bacteria of the same strains also suggested there is as much 0-chain material in the FOR strains as in the parent strain. The data suggest that the FOR strains are the result of a mutation in the synthesis of the core, leaving few, if any, completed cores accessible to the FO phage.