Abstract: The present invention relates to a serum for passive immunization of fish against furunculosis caused by Aeromonas salmonicida, which comprises antibodies against A. salmonicida isolated from fish naturally infected with furunculosis in an intraperitoneal injectable carrier. There is also provided a method for preparing a serum for passive immunization of fish against furunculosis caused by Aeromonas salmonicida, comprising the steps of: a) administering to a mammal selected from the group consisting of bovine, ovine, caprine and equine a sufficient amount of A. salmonicida to produce antibodies; b) collecting from the mammal the serum containing the antibodies against A.