An indirect enzyme-linked immunosorbent assay (ELISA) based on the lipopolysaccharide (LPS) of Salmonella enteritidis phage type 4, was developed for the detection of antibodies to salmonella. Sera and yolks from chickens infected experimentally with S enteritidis showed strong positive reactions. Crossreactions occurred with sera from chickens inoculated with S typhimurium or S gallinarum. Cross-reactions were weak with sera from chickens infected with five strains of other Enterobacteriaceae. The ELISA was tested with sera and yolks from commercial poultry flocks which were bacteriologically negative for salmonella or infected with salmonella serotypes belonging to serogroup D or to other serogroups. The serological reactions were strong in most flocks infected with S enteritidis and were weaker in flocks infected with S typhimurium. In some flocks infected with these serotypes no antibodies were detected. The correct setting of the cut-off value of the optical density in the ELISA makes it possible to discriminate between chickens which are infected with S enteritidis and chickens which are not infected with S enteritidis.