Abstract: The invention presents a composition for controlling mastitis in ruminants, a method for preparing the composition and a method of treatment of ruminants. The composition contains one or more soluble exoantigens of gram-positive bacteria involved in mastitis, or immunochemical homologues thereof, in a modified form which creates or enhances their ability to induce acquired mammary hypersensitivity. Said composition may be used as a vaccine or as part of a vaccine for ruminants.