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Abstract. Polysaccharides derived from Salmonella typhimurium lipopolysaccharide (LPS) representing the O-antigen factors 1, 4, 5, and 12 and the O-antigen factors 6 and 7 from Salmonella choleraesuis LPS were derivatized with the photoreactive compound anthraquinone and subsequently covalently coupled to microtiter polystyrene plates by ultraviolet irradiation. Both polysaccharide antigens could be coupled simultaneously to the same microtiter plate. The coated surface was used in indirect ELISA for the determination of serum antibodies from pigs infected with bacteria of the two Salmonella groups and from uninfected pigs. This ELISA proved itself by having a good long-term durability and a high degree of reproducibility, including low dayto-day variations and low interplate variations. Furthermore, the ELISA showed good specificity and sensitivity when data were compared with the optical density levels of a panel of pig sera as determined by a conventional ELISA on the basis of passive coating of the two Salmonella LPS antigens (the mix-ELISA). The covalent anthraquinone mix-ELISA shows promise as a stable and durable alternative to the existing conventional ELISA for serological surveillance of Salmonella infections in pigs.Bacterial lipopolysaccharides (LPSs) are important serotype-defining antigens of gram-negative bacteria, 17 and they are often, therefore, used as antigens in serological tests for the detection of serotype-specific antibodies. 2,10 Lipopolysaccharide consists of the hydrophobic lipid A part and the hydrophilic polysaccharide (PS) part. The latter can be further subdivided into a constant core region roughly similar in all Enterobacteriaceae and the variable O-antigenic PS specific for the bacterial serotype. 4,13 Lipopolysaccharide is anchored to the outer bacterial membrane by lipid A, whereas the highly immunoreactive PS part is exposed and able to interact with the immune system of the infected host. 17 In conventional solid-phase immunoassays (e.g., ELISA) for detection of anti-LPS antibodies, purified LPS is passively adsorbed to the plastic surface by hydrophobic interactions between the lipid A and the plastic surface, leaving the antigenic PS exposed to the solvent and freely accessible to the antibodies. However, coating by passive adsorption is strongly dependent on the overall amphiphilicity of the LPS, being in turn dependent on the specific type of LPS, and because purified LPS is heterogeneous because of the From the Danish Veterinary Laboratory,

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