Enzyme‐linked immunosorbent assays (ELISAs) were developed for the detection of antibodies toMycoplasma synoviae,infectious bronchitis virus (IBV)Pasteurella multocida,infectious bursal disease virus (IBDV), reovirus, egg‐drop syndrome (EDS) and reticuloendotheliosis virus (REV). Purified antigenic preparations derived from the various pathogens were used to coat microtitre plates. A non‐specific factor in some serum samples bound to many of the antigen‐coated plates. Such non‐specific binding was shown to be related to the age of the chicken and to immunization schedules.