Four EIA (enzyme immunoassay) test systems for the detection of salmonella O-antigens in various biological tissues were studied. To find the optimum test system two types of affinity purified antibodies were used to coat the microtitre plates: (i) monospecific antibodies isolated on immunoadsorbent bearing synthetic O-antigen factor 4 (O:4; salmonella serogroup B) as ligand, (ii) antibodies specific for lipopolysaccharide (LPS) B isolated from the IgG fraction of hyperimmune sera. The use of affinity purified antibodies led to an increase in the sensitivity of 'sandwich' EIA by an order of magnitude and to improved specificity. Competitive EIA was ten to twenty times less sensitive. It is demonstrated for the first time that salmonella O-antigens can be detected in the sera of animals within a day after challenge. For patients with salmonellosis, O-antigen could be detected after the fifth day of illness, but in the urine and faeces only (not in the blood serum), in 30%-70% of cases. This substantially improves the identification of salmonellosis from among other enteric infections. In competitive EIA, inhibition of standard antibodies by the sera under study was caused not by the presence of O-antigen but by antibodies homologous to the coating antigen. This resulted in "blocking' of the latter and led to false-positive results. The results obtained enable the optimum EIA technique to be selected to improve the serological diagnosis of salmonellosis with both synthetic salmonella O-antigens and LPS.