The phage adsorption ability and serological specificity of different Salmonella strains having either complete or leaky mutations in their lipopolysaccharide (LPS) synthesis were compared, together with their genotype and sugar composition, to provide a set of standards relating these parameters to LPS structure. Strains that had T1-specific side chains in their LPS, both with or without O side chains, were examined to learn more about the organization of these two side chains in the LPS and a possible competition between them. It was found that (i) adsorption of O-specific antibodies was a very sensitive test for the presence of even very small amounts of O-specific structures, (ii) that phage P22 adsorption was dependent on the presence of a nearly complete O side chain complement, and both long and numerous O side chains were required, and (iii) that the adsorption of the phages FO (Felix O-1), 6SR, and Br2, which attach to structures in the LPS core, was a sensitive indicator of any defect in O-antigen synthesis, and well developed O side chains blocked their attachment efficiently. Semirough (SR) strains with only one O-specific repeating unit per side chain adsorbed FO efficiently, whereas the access of the 6SR and Br2 phages to their receptors was blocked. Strains with T1 side chains adsorbed the FO and 6SR phages efficiently, whereas the adsorption of the Br2 phage was blocked to a large extent. The phage adsorption of four S, T1 strains (with both O and T1 side chains) showed that, as the amount of O side chain material increased, there was a reduction of the adsorption of phages in the following order: 6SR, Br2, and FO. P22 attachment appeared with the increase of O side chains. The LPS composition of these strains revealed a 10-fold reduction of the O-specific structures compared to the smooth parent strain, whereas the amount of T1-specific material was the same as in T1 strains. The short O side chains of a SR, T1 strain were, however, not reduced in number, suggesting that the apparent competition between O and T1 side chains may not be a competition for available sites in the LPS.