Interaction between phage P22 and phenol-water extracted lipopolysaccharides from sensitive Salmonella bacteria belonging to serogroups A, B and Di results in hydrolysis of the alpha-L-rhamnosyl linkages within the tetrasaccharide repeating unit of the O-antigenic polysaccharide chain. These O-antigens have identical structures except for the nature of the 3,6-dideoxy-hexosyl group linked to O-3 of the D-mannosyl residue. Removal of the dideoxysugar, or periodate oxidation followed by borohydride reduction of the L-rhamnosyl residue made the O chain resistant to the endo-rhamnosidase. Substitution of the D-galactosyl residue at O-4, but not at O-6, with an alpha-D-glucosyl group was compatible with hydrolysis. A number of Klebsiella pneumoniae and Shigella flexneri lipo- or capsular polysaccharides containing chain L-rhamnosyl residues were tested but none was sensitive to the P22 endo-rhamnosidase. The substrate specificity of the endo-rhamnosidase parallels the lytic specificity of the phage which suggests that the initial step in phage P22 infection is a P22 tail enzyme O-antigen substrate interaction. The main product of the hydrolysate was octa-, dodeca- and hexadecasaccharides. Treatment of phage FO resistant smooth strains of S. typhimurium with P22 tails removed O polysaccharide chains and made previously 'hidden' FO receptors accessible to the phage.