The extraction of lipopolysaccharides (LPS) from smooth strains of Salmonella typhimurium for structural studies using methylation analysis is preferably done by hot phenol-water method on a cell-wall preparation obtained by mechanical disintegration of γ-irradiated bacteria. The LPS obtained by this method was less contaminated than LPS extracted by hot phenol-water from acetone dried bacteria or bacteria pretreated with 1 per cent formaldehyde. LPS for structural analysis from rough mutants is, due to its lipophilic character, preferably extracted with a mixture of phenol, chloroform and petroleum ether. The presence of the precursor of the O side-chain, LI-hapten, in the different LPS- and supernatant fractions was assessed by gel precipitation tests. The LPS forms a line near the antigen well whereas the low molecular weight hapten forms a line near the antiserum well. The hapten was not formed in excess in the smooth strain S. typhimurium LT2. In the rough mutant studied, S. typhimurium SL 733, O-antigenic material was found in both the LPS-preparation (probably as a covalently linked O side-chain) and the supernatant (as a hapten).