Immunization of rabbits with the synthetic disaccharide-protein conjugate, tyvelose 1 leads to alpha 3 mannose 1 leads to bovine serum albumin (TM-BSA) in Freund's complete adjuvant, gave rise to antibodies directed against both the disaccharide hapten and the carrier protein. The hapten antibodies were specific for O-antigen 9 of Salmonella and were superior in terms of specificity to conventional O factor 9 antibodies, as judged by passive haemagglutination and enzyme-linked immunosorbent assay (ELISA). A complement dependent bactericidal assay demonstrated that the TM-BSA antibodies had bacteriolytic activity also directed against the O-antigen 9 Salmonella. The high specificity was evident in both the IgG and IgM antibody classes. In the bactericidal assay the IgM antibodies were more active than antibodies of the IgG class. The results suggested that IgG antibodies in hyperimmune serum blocked the complement dependent bactericidal activity of IgM antibodies.