Due to the already demonstrated specificity and usefulness of Listeria intracellular antigen in paraffin-embedded tissues for the direct diagnosis, we have used an indirect immunoperoxidase technique for the serological diagnosis of listeriosis in man, sheep and rabbit. This technique has been compared with other immunoperoxidase techniques (PAP) using the same antigen, and with other serlogical techniques--ELISA and microplate agglutination test--using two kinds of Listeria antigens, live formalinized and heat at 100 degrees C for one hour whole cells.