The O-antigenic polysaccharide chain of phenol-water extracted Salmonella typhimurium (O antigens 4, 12) lipopolysaccharide was enzymatically cleaved by phage P22 endorhamnosidase. An octasaccharide with the structure was isolated and shown to retain the O-antigen 4 specificity of the native polysaccharide. After oxidation of the terminal reducing rhamnose residue to the corresponding aldonic acid, the octasaccharide was covalently linked to bovine serum albumin (OLS-BSA) by use of a water-soluble carbodiimide. The resulting conjugate showed O-antigen 4 specificity in enzyme-linked immunosorbent assay (ELISA) and passive hemagglutination inhibition tests. Immunization of rabbits with the OLS-BSA conjugate gave rise to antibodies directed toward both the octasaccharide and the carrier protein. ELISA titration with synthetic disaccharide-protein conjugates as antigens revealed that the antibody titer against the mannose-rhamnose structure was higher than against the abequose-mannose structure. In rabbits immunized with heat-killed whole bacteria the titers against the two disaccharides were equal. The reason for this difference is not obvious. It is evident, however, that the OLS-BSA conjugate elicited in rabbits O-antibodies with the same specificity as whole bacteria.