Lipopolysaccharide (LPS) isolated from the pyocin sensitive R form strain of Salmonella minnesota F6 (chemotype Rd1) inhibited the activity of bacteriophage tail-like pyocin P1 whereas no inhibition occurred with LPS prepared from the pyocin resistant S form of S. minnesota. Subunits of lipopolysaccharide obtained by treatment with sodium deoxycholate and the polysaccharide fraction of the lipopolysaccharide obtained by acid hydrolysis were shown to be still active whereas lipid A fraction had no pyocin neutralizing activity. The (KDO)3-hepI-hepII unit, which terminates the lipopolysaccharide of S. minnesota F6 was, therefore, suggested to determine the specificity of the pyocin P1 receptor.