The present invention relates to (1) an enzyme-linked immunosorbent assay (ELISA) for detection in milk of antibodies of any isotype which are specific for Staphylococcus aureus proteins in molecular weights ranging from 18,000 to 26,000 daltons, (2) a process for production and purification of said proteins, (3) a method of performing said ELISA utilizing said proteins and (4) use of said ELISA for detection of intramammary infection by S. aureus.